Extending Heka

The core of the Heka engine is written in the Go programming language. Heka supports six different types of plugins (inputs, splitters, decoders, filters, encoders, and outputs), which are also written in Go. This document will try to provide enough information for developers to extend Heka by implementing their own custom plugins. It assumes a small amount of familiarity with Go, although any reasonably experienced programmer will probably be able to follow along with no trouble.


Heka also supports the use of security sandboxed Lua code for implementing the core logic of every plugin type except splitters. This document only covers the development of Go plugins. You can learn more about sandboxed plugins in the Sandbox section.


You should be familiar with the Glossary terminology before proceeding.


Each Heka plugin type performs a specific task. Inputs receive input from the outside world and inject the data into the Heka pipeline. Splitters slice the input stream into individual records. Decoders turn binary data into Message objects that Heka can process. Filters perform arbitrary processing (aggregation, collation, monitoring, etc.) of Heka message data. Encoders serialize Heka messages into arbitrary byte streams. Outputs send data from Heka back to the outside world. Each specific plugin has some custom behaviour, but it also shares behaviour w/ every other plugin of that type. A UDPInput and a TCPInput listen on the network differently, and a LogstreamerInput (reading files off the file system) doesn’t listen on the network at all, but all of these inputs need to interact w/ the Heka system to access data structures, gain access to decoders to which we pass our incoming data, respond to shutdown and other system events, etc.

To support this all Heka plugins except encoders actually consist of two parts: the plugin itself, and an accompanying “plugin runner”. Inputs have an InputRunner, splitters have a SplitterRunner, decoders have a DecoderRunner, filters have a FilterRunner, and Outputs have an OutputRunner. The plugin itself contains the plugin-specific behaviour, and is provided by the plugin developer. The plugin runner contains the shared (by type) behaviour, and is provided by Heka. When Heka starts a plugin, it first creates and configures a plugin instance of the appropriate type, then it creates a plugin runner instance of the appropriate type, passing in the plugin.

For inputs, filters, and outputs, there’s a 1:1 correspondence between sections specified in the config file and running plugin instances. This is not the case for splitters, decoders and encoders, however. Configuration sections for splitter, decoder and encoder plugins register possible configurations, but actual running instances of these types aren’t created until they are used by input or output plugins.

Plugin Configuration

Heka uses TOML as its configuration file format (see: Configuring hekad), and provides a simple mechanism through which plugins can integrate with the configuration loading system to initialize themselves from settings in hekad’s config file.

The minimal shared interface that a Heka plugin must implement in order to use the config system is (unsurprisingly) Plugin, defined in pipeline_runner.go:

type Plugin interface {
    Init(config interface{}) error

During Heka initialization an instance of every plugin listed in the configuration file will be created. The TOML configuration for each plugin will be parsed and the resulting configuration object will be passed in to the above specified Init method. The argument is of type interface{}. By default the underlying type will be *pipeline.PluginConfig, a map object that provides config data as key/value pairs. There is also a way for plugins to specify a custom struct to be used instead of the generic PluginConfig type (see Custom Plugin Config Structs). In either case, the config object will be already loaded with values read in from the TOML file, which your plugin code can then use to initialize itself. The input, filter, and output plugins will then be started so they can begin processing messages. The splitter, decoder, and encoder instances will be thrown away, with new ones created as needed when requested by input (for splitter and decoder) or output (for encoder) plugins.

As an example, imagine we’re writing a filter that will deliver messages to a specific output plugin, but only if they come from a list of approved hosts. Both ‘hosts’ and ‘output’ would be required in the plugin’s config section. Here’s one version of what the plugin definition and Init method might look like:

type HostFilter struct {
    hosts  map[string]bool
    output string

// Extract hosts value from config and store it on the plugin instance.
func (f *HostFilter) Init(config interface{}) error {
    var (
        hostsConf  interface{}
        hosts      []interface{}
        host       string
        outputConf interface{}
        ok         bool
    conf := config.(pipeline.PluginConfig)
    if hostsConf, ok = conf["hosts"]; !ok {
        return errors.New("No 'hosts' setting specified.")
    if hosts, ok = hostsConf.([]interface{}); !ok {
        return errors.New("'hosts' setting not a sequence.")
    if outputConf, ok = conf["output"]; !ok {
        return errors.New("No 'output' setting specified.")
    if f.output, ok = outputConf.(string); !ok {
        return errors.New("'output' setting not a string value.")
    f.hosts = make(map[string]bool)
    for _, h := range hosts {
        if host, ok = h.(string); !ok {
            return errors.New("Non-string host value.")
        f.hosts[host] = true
    return nil

(Note that this is a contrived example. In practice, messages are routed to outputs using the Message Matcher Syntax.)

Restarting Plugins

If your plugin supports being restarted and either fails to initialize properly at startup, or fails during Run with an error (perhaps because a network connection dropped, a file became unavailable, etc) then Heka will attempt to reinitialize and restart it up until the specified max_retries value.

If the failure continues beyond the maximum number of retries, or if the plugin didn’t support restarting in the first place, then Heka will either shut down or, if the plugin is an input, filter or an output with the can_exit setting set to true, the plugin will be removed from operation and Heka will continue to run.

To add restart support to your plugin, you must implement the Restarting interface defined in the config.go file:

type Restarting interface {

The CleanupForRestart method will be called when the plugin’s main run method exits, a single time. This allows you a place to perform any additional cleanup that might be necessary before attempting to reinitialize the plugin. After this, the runner will repeatedly call the plugin’s Init method until it initializes successfully. It will then resume running it unless it exits again at which point the restart process will begin anew.

Custom Plugin Config Structs

In simple cases it might be fine to get plugin configuration data as a generic map of keys and values, but if there are more than a couple of config settings then checking for, extracting, and validating the values quickly becomes a lot of work. Heka plugins can instead specify a schema struct for their configuration data, into which the TOML configuration will be decoded.

Plugins that wish to provide a custom configuration struct should implement the HasConfigStruct interface defined in the config.go file:

type HasConfigStruct interface {
    ConfigStruct() interface{}

Any plugin that implements this method should return a struct that can act as the schema for the plugin configuration. Heka’s config loader will then try to decode the plugin’s TOML config into this struct. Note that this also gives you a way to specify default config values; you just populate your config struct as desired before returning it from the ConfigStruct method.

Let’s look at the code for Heka’s UdpOutput, which delivers messages to a UDP listener somewhere. The initialization code looks as follows:

// This is our plugin struct.
type UdpOutput struct {
    conn net.Conn

// This is our plugin's config struct
type UdpOutputConfig struct {
    // Network type ("udp", "udp4", "udp6", or "unixgram"). Needs to match the
    // input type.
    Net string
    // String representation of the address of the network connection to which
    // we will be sending out packets (e.g. "").
    Address string
    // Optional address to use as the local address for the connection.
    LocalAddress string `toml:"local_address"`

    // Maximum size of message, plugin drops the data if it exceeds this limit.
    MaxMessageSize int `toml:"max_message_size"`

// Provides pipeline.HasConfigStruct interface.
func (o *UdpOutput) ConfigStruct() interface{} {
    return &UdpOutputConfig{
        Net: "udp",

        // Defines maximum size of udp data for IPv4.
        MaxMessageSize: 65507,

// Initialize UDP connection
func (o *UdpOutput) Init(config interface{}) (err error) {
    o.UdpOutputConfig = config.(*UdpOutputConfig) // assert we have the right config type

    if o.UdpOutputConfig.MaxMessageSize < 512 {
                return fmt.Errorf("Maximum message size can't be smaller than 512 bytes.")

    if o.Net == "unixgram" {
        if runtime.GOOS == "windows" {
            return errors.New("Can't use Unix datagram sockets on Windows.")
        var unixAddr, lAddr *net.UnixAddr
        unixAddr, err = net.ResolveUnixAddr(o.Net, o.Address)
        if err != nil {
            return fmt.Errorf("Error resolving unixgram address '%s': %s", o.Address,
        if o.LocalAddress != "" {
            lAddr, err = net.ResolveUnixAddr(o.Net, o.LocalAddress)
            if err != nil {
                return fmt.Errorf("Error resolving local unixgram address '%s': %s",
                    o.LocalAddress, err.Error())
        if o.conn, err = net.DialUnix(o.Net, lAddr, unixAddr); err != nil {
            return fmt.Errorf("Can't connect to '%s': %s", o.Address,
    } else {
        var udpAddr, lAddr *net.UDPAddr
        if udpAddr, err = net.ResolveUDPAddr(o.Net, o.Address); err != nil {
            return fmt.Errorf("Error resolving UDP address '%s': %s", o.Address,
        if o.LocalAddress != "" {
            lAddr, err = net.ResolveUDPAddr(o.Net, o.LocalAddress)
            if err != nil {
                return fmt.Errorf("Error resolving local UDP address '%s': %s",
                    o.Address, err.Error())
        if o.conn, err = net.DialUDP(o.Net, lAddr, udpAddr); err != nil {
            return fmt.Errorf("Can't connect to '%s': %s", o.Address,

In addition to specifying configuration options that are specific to your plugin, it is also possible to use the config struct to specify default values for any common configuration options that are processed by Heka, such as the synchronous_decode option available to Input plugins, or the ticker_interval, message_matcher, and buffering values that are available to filter and output plugins. If a config struct contains a uint attribute called TickerInterval, that will be used as a default ticker interval value (in seconds) if none is supplied in the TOML. Similarly, if a config struct contains a string attribute called MessageMatcher, that will be used as the default message routing rule if none is specified in the configuration file.

There is an optional configuration interface called WantsName. It provides a plugin access to its configured name before the runner has started. The SandboxFilter plugin uses the name to locate/load any preserved state before being run:

type WantsName interface {
    SetName(name string)

There is also a similar WantsPipelineConfig interface that can be used if a plugin needs access to the active PipelineConfig or GlobalConfigStruct values in the ConfigStruct or Init methods:

type WantsPipelineConfig interface {
    SetPipelineConfig(pConfig *pipeline.PipelineConfig)

Note that, in the case of inputs, filters, and outputs, these interfaces only need to be implemented if you need this information before the plugin is started. Once started, the plugin runner and a plugin helper will be passed in to the Run or Prepare method, which make the plugin name and PipelineConfig struct available in other ways.


Input plugins are responsible for acquiring data from the outside world and injecting this data into the Heka pipeline. An input might be passively listening for incoming network data or actively scanning external sources (either on the local machine or over a network). The input plugin interface is:

type Input interface {
    Run(ir InputRunner, h PluginHelper) (err error)

The Run method is called when Heka starts and, if all is functioning as intended, should not return until Heka is shut down. If a condition arises such that the input can not perform its intended activity it should return with an appropriate error, otherwise it should continue to run until a shutdown event is triggered by Heka calling the input’s Stop method, at which time any clean-up should be done and a clean shutdown should be indicated by returning a nil error.

Inside the Run method, an input typically has three primary responsibilities:

  1. Access some data or data stream from the outside world.
  2. Provide acquired data or stream to a SplitterRunner for record extraction and further delivery.
  3. (optional) Provide a “pack decorator” function to the SplitterRunner to populate the message object with any input-specific information.

The details of the first step are clearly entirely defined by the plugin’s intended input mechanism(s). Plugins can (and should!) spin up goroutines as needed to perform tasks such as listening on a network connection, making requests to external data sources, scanning machine resources and operational characteristics, reading files from a file system, etc.

For the second step, you need to get a SplitterRunner to which you can feed your incoming data. This is available through the InputRunner’s NewSplitterRunner method. NewSplitterRunner takes a single string argument called token. This token is used to differentiate multiple SplitterRunner instances from each other. If you have a simple input plugin that only needs a single SplitterRunner, you can just pass an empty string (i.e. sr := ir.NewSplitterRunner("")). In more complicated scenarios you might want multiple SplitterRunners, say one per goroutine, in which case you should pass a unique identifier string in to each NewSplitterRunner call.

Splitting records efficiently is a surprisingly complicated process so the SplitterRunner interface has a number of methods:

type SplitterRunner interface {
    SetInputRunner(ir InputRunner)
    Splitter() Splitter
    SplitBytes(data []byte, del Deliverer) error
    SplitStream(r io.Reader, del Deliverer) error
    GetRemainingData() (record []byte)
    GetRecordFromStream(r io.Reader) (int, []byte, error)
    DeliverRecord(record []byte, del Deliverer)
    KeepTruncated() bool
    UseMsgBytes() bool
    SetPackDecorator(decorator func(*PipelinePack))

Don’t let this scare you, however. SplitterRunner’s expose some internal workings to be able to support advanced uses, but in most cases you only need to deal with a few of the exposed methods. Specifically, you care about either SplitStream or SplitBytes, possibly about SetPackDecorator and UseMsgBytes, and you need to call Done when the Splitter is no longer needed.

First we’ll examine the “Split” methods. As mentioned above, you’ll typically only want to use one or the other. Deciding which you want is straightforward. If your mechanism for getting data from the outside world is a stream object (an io.Reader, in Go terms), then you’ll want SplitStream. If not and you just end up with a byte slice of binary data, then you’ll want SplitBytes.

Note that both SplitStream and SplitBytes ask for a Deliverer object as their second argument. Again, in simple cases you don’t need to worry about this. If you’re only using a single SplitterRunner, you can just pass in nil and Heka will take care of delivering the message to a decoder and/or the message router appropriately. If you’re using multiple goroutines (and therefore multiple SplitterRunners), however, you’ll typically want multiple Deliverers, too. This is especially important if you want each separate goroutine to have its own Decoder, so decoding can happen in parallel, delegated to multiple cores on a single machine.

Like SplitterRunners, Deliverers are obtained from the InputRunner, using the NewDeliverer method. And, like SplitterRunners, NewDeliverer takes a single string identifier argument, which should be unique for each requested deliverer. Usually a single SplitterRunner will be using a single Deliverer, and the same token identifier will be used for each. You can see an example of this in the TcpInput’s handleConnection code snippet a bit further down this page. It’s also important to keep in mind that both SplitterRunners and Deliverers expose a Done method that should be called by the input plugin’s code whenever the resource is no longer being used so Heka can free it up appropriately.

If you’re using SplitBytes, then you’ll want to call it each time you have a new payload of data to process. It will return the number of bytes successfully consumed from the provided slice, and any relevant errors occurred while processing. It is up to the calling code to decide what to do in error cases, or when all of the data isn’t consumed.

If you’re using SplitStream, then the SplitStream call will block for as long as it is consuming data. When data processing pauses or stops, SplitStream will exit and return control back to the input, returning either nil or any relevant errors. Typically if nil is returned, you’ll want to call SplitStream again to continue processing the stream. Code such as the following is a common idiom:

var err error
for err == nil {
    err = sr.SplitStream(r, nil)

Any errors encountered while processing the stream, including io.EOF, will be returned from the SplitStream call. It is up to the input code to decide how to proceed.

Finally, we’re ready for the third step, providing a “pack decorator” function to the SplitterRunner. Sometimes an input plugin would like to populate a Heka message with information specific to the input mechanism. The TcpInput, for instance, often wants to store the remote address of the TCP connection as a message’s Hostname field. Any provided pack decorator function will be called immediately before the PipelinePack is passed on for delivery, allowing the input to mutate the pack’s Message struct as desired. The TcpInput code that uses this feature looks like so:

func (t *TcpInput) handleConnection(conn net.Conn) {
    raddr := conn.RemoteAddr().String()
    host, _, err := net.SplitHostPort(raddr)
    if err != nil {
        host = raddr

    deliverer :=
    sr :=

    defer func() {

    if !sr.UseMsgBytes() {
        packDec := func(pack *PipelinePack) {

The if !sr.UseMsgBytes() check before the SetPackDecorator call deserves some explanation. Generally Heka receives input data in one of two flavors. The first is standalone data, usually text, such as log files loaded from the file system using a LogstreamerInput. This data is stored within a Message struct, usually as the payload. Most decoder plugins, then, will expect to find the raw input data in the Message payload, and will parse this data and mutate the Message struct with extracted data.

The second flavor of input data is a binary blob, usually protocol buffers encoded, representing an entire Heka message. Clearly it doesn’t make much sense to store data representing a serialized Message struct within a Message struct, since it would overwrite itself upon deserialization. For this reason, PipelinePacks have a MsgBytes attribute that is used as a buffer for storing binary data that will be converted to a message. Certain decoder plugins, most notably the ProtobufDecoder, will expect to find input data in the pack.MsgBytes buffer, and will use this to create a new Message struct from scratch.

Splitters can specify via a config setting whether the data records they parse should be placed in the message payload of an existing Message struct or in the MsgBytes attribute of the enclosing PipelinePack, depending on what the accompanying decoder plugin expects. The UseMsgBytes method on the SplitterRunner will return true if the contained splitter plugin is putting the data in the MsgBytes buffer, or false if it is putting the data in the Message’s Payload field.

Now we can understand why the TcpInput is checking this before setting the pack decorator. When UseMsgBytes returns true, then the Message struct on that pack is going to be overwritten when decoding happens. There’s not much value in setting the Hostname field when it’s going to be clobbered shortly afterward.

Okay, that covers most of what you need to know about developing your own Heka input plugins. There’s one important final possibility to consider, however. In some cases, an input might fail to retrieve any data at all, so it has nothing to hand to the Splitter. Even so, it might still want to deliver a message containing information about the data retrieval failure itself. The HttpInput does this when an HTTP request fails completely due to network or other errors, for instance.

When this happens the input must obtain a fresh PipelinePack, manually populate the contained Message struct, and manually hand it over for delivery. Here’s the snippet in the HttpInput code that does this:

resp, err := httpClient.Do(req)
responseTime := time.Since(responseTimeStart)
if err != nil {
    pack := <

As you can see, the pattern is simple. The PipelinePack supply is exposed via a channel provided by the InputRunner’s InChan method, so we pull from this channel to get a fresh pack. Then we populate the Message struct with any relevant data we want to include, and we finish up by passing the pack in to the InputRunner’s Deliver method for delivery. If we were using separate Deliverers, then we would call the Deliver method on the relevant Deliverer instance instead of on the InputRunner.

One important detail about this pattern, however: if for any reason your plugin should pull a PipelinePack off of the input channel and not end up passing it on to one of the Deliver methods, you must call pack.Recycle(nil) to free the pack up to be used again. Failure to do so will eventually deplete the pool of PipelinePacks and will cause Heka to freeze.


In contrast to the relatively complicated SplitterRunner interface that is discussed in the Inputs section above, the actual Splitter plugins themselves are very simple. The basic Splitter interface consists of a single method:

// Splitter plugin interface type.
type Splitter interface {
    FindRecord(buf []byte) (bytesRead int, record []byte)

The job of the FindRecord method is straightforward. It should scan through the provided byte slice, from the beginning, looking for any delimiters or appropriate indicators of a record boundary. It returns two values, the number of bytes consumed from the input buffer, and a slice that represents any record that was found. The bytesRead value should always be returned, whether a record slice is returned or not. If the entire buffer was scanned but no record was found, for instance, then bytesRead should be len(buf).

Note that when a record is discovered, the returned slice can (and should, if possible) be a subsection of the input buffer. It’s recommended that FindRecord not do any unnecessary copying of the input data.

In many cases this is all that is required of a splitter plugin. In some situations, however, records may include some headers and/or framing of some sort, and additional processing of those headers might be called for. For instance, Heka’s native Stream Framing can embed HMAC authenticated message signing information in the message header, and the splitter needs to be able to decide whether or not the authentication is valid. For this reason, splitter plugins can implement an additional UnframingSplitter interface:

// UnframingSplitter is an interface optionally implemented by splitter
// plugins to remove and process any record framing that may have been used by
// the splitter.
type UnframingSplitter interface {
    UnframeRecord(framed []byte, pack *PipelinePack) []byte

The FindRecord method of an UnframingSplitter should return the full record, frame and all. Heka will then pass each framed record into the UnframeRecord method, along with the PipelinePack into which the record will be written. UnframeRecord should then extract the record framing, process it as needed, and return a byte slice containing the unframed record that is remaining. As with FindRecord, copying the data isn’t necessary, the unframed record can safely refer to a subslice of the original framed record.

If the splitter examines the headers and decides that a given record is for some reason not valid, such as for the use of an incorrect authentication key, then it should return nil instead of the contained record. Additionally, signing information can be written to the PipelinePack’s Signer attribute, and this will be honored by the message_signer config setting available to filter and output plugins.

Note that if UnframeRecord returns nil it does not need to call pack.Recycle(nil). Heka will recognize that the pack isn’t going to be used and will recycle it itself.


Decoder plugins are responsible for converting raw bytes containing message data into actual Message struct objects that the Heka pipeline can process. As with inputs and splitters, the Decoder interface is quite simple:

type Decoder interface {
    Decode(pack *PipelinePack) (packs []*PipelinePack, err error)

There are three additional optional interfaces a decoder might decide to implement. The first provides the decoder access to its DecoderRunner object when it is started:

type WantsDecoderRunner interface {
    SetDecoderRunner(dr DecoderRunner)

The second provides a notification to the decoder when the DecoderRunner is exiting:

type WantsDecoderRunnerShutdown interface {

Understanding the third optional interface requires a bit of context. Heka’s PipelinePack structs contain a Message attribute, which points to the actual instantiated Message struct, and a MsgBytes attribute, which is generally used to hold the protocol buffer encoding of the Message struct. Whenever a message is injected into the message router, Heka will protobuf encode that message and store the result in the MsgBytes attribute, also setting the pack’s TrustMsgBytes attribute flag to true.

In some cases, however, a protobuf encoding of the message is already available. For instance, when a message is received in protobuf format and is not further mutated, as in the case when an input is using a single ProtobufDecoder, then the original incoming data is already a valid protobuf encoding. Any decoder that might already have access to or generate a valid protobuf encoding for the resulting message should implement the EncodesMsgBytes interface:

type EncodesMsgBytes interface {
    EncodesMsgBytes() bool

Heka will check for this method at startup and, if it exists, it will assume that the decoder plugin may populate the MsgBytes data with the encoded message data, and that it will set pack.TrustMsgBytes to true if it does.

A decoder’s Decode method should extract raw message data from the provided pack. Depending on the nature of the decoder, this might be found either in the MsgBytes attribute of the PipelinePack, or in the contained Message struct’s Payload field. Then it should try to deserialize and/or parse this raw data, using the contained information to overwrite or populate the pack’s Message struct.

If the decoding / parsing operation concludes successfully then Decode should return a slice of PipelinePack pointers and a nil error value. The first item in the returned slice (i.e. packs[0]) should be the pack that was passed in to the method. If the decoding process needs to produce more than one output pack, additional ones can be obtained from the DecoderRunner’s NewPack method, and they should be appended to the returned slice of packs.

If decoding fails for any reason, then Decode should return a nil value for the PipelinePack slice and an appropriate error value. Returning an error will cause Heka to log an error message about the decoding failure. Additionally, if the associated input plugin’s configuration set the send_decode_failure value to true, the message will be tagged with decode_failure and decode_error fields and delivered to the router.

About Message Mutation

All of the above plugin types (i.e. inputs, splitters, and decoders) come before the router in Heka’s pipeline, and therefore they may safely mutate the message struct. Once a pack hits the router, however, it is no longer safe to mutate the message, because a) it might be concurrently processed by more that one filter and/or output plugin, leading to race conditions; and b) a protobuf encoding of the message will be stored in the pack.MsgBytes attribute, and mutating the message will cause this encoding to become out of sync with the actual message.

Filter, encoder, and output plugins should never mutate Heka messages. Sandbox plugins will prevent you from doing so. SandboxEncoders, in particular, expose the write_message API that appears to mutate a message, but it actually creates a new message struct rather than modifying the existing one (i.e. copy-on-write). If you implement your own filter, encoder, or output plugins in Go, you must take care to honor this requirement and not mutate any PipelinePack or Message structs.

New in version 0.10.

Transitional Filter / Output APIs for v0.10 Only

Heka’s APIs for filter and output plugins have changed dramatically from version 0.9 to version 0.10, to be able to efficiently support disk buffering. The new (and future-proof) APIs for these plugin types are described below. For the 0.10.X series of Heka releases, however, a very slightly modified version of the older APIs will be available. This allows Heka users with a significant number of existing filter and output plugins to get them working with a minimal amount of effort.

All filter and output plugin code should soon be upgraded to support the new API code, however, because a) the older APIs incur a considerable performance penalty when disk buffering is in use, and b) the older APIs are deprecated and will be removed from Heka entirely in future releases.


Filter plugins are the message processing engine of the Heka system. They are used to examine and process message contents, and trigger events based on those contents in real time as messages are flowing through the Heka system.

Relevant Interfaces

There are three interfaces related to filter plugin implementations. The first of these is the Filter interface:

type Filter interface {
    Prepare(fr FilterRunner, h PluginHelper) (err error)

The Filter interface provides two methods. The first, Prepare, will be called by Heka to finalize initialization and start any needed additional goroutines before message processing happens. The provided FilterRunner and PluginHelper interfaces give the filter access to the relevant Heka APIs. Any error returned will indicate that the initialization failed, preventing any messages from being delivered to the filter and possibly causing Heka to shut down, depending on the plugin’s can_exit value. The CleanUp method will be called after message processing has stopped to allow the filter an opportunity to clean up any resources that might need to be freed when exiting.

The second relevant interface for filter plugins is the MessageProcessor interface. All filter plugins must implement this interface.

The third relevant interface is TickerPlugin. The TickerPlugin interface is not strictly required to be implemented by every filter plugin, but most of them will want to do so, and the failure to implement it will mean that your filter will not support the ticker_interval config setting.


All filter plugins can be configured to support disk buffering, so they should regularly call the FilterRunner’s UpdateCursor method as described here to advance the buffer’s cursor in cases where buffering is used.

Message Injection

Filter plugins will often need to create new messages that should be injected into Heka’s router for further processing by other filter or output plugins, from either the ProcessMessage method or (more often) from TimerEvent. In either case, the process is the same.

To generate new messages, your filter must call PluginHelper.PipelinePack(msgLoopCount int). The msgloopCount value to be passed in should be obtained from the MsgLoopCount value on the pack that you’re already holding, or zero if called from within TimerEvent. The PipelinePack method returns two values, the first a *PipelinePack and the second an error. If all goes well, you’ll get a pack ready for you to populate and a nil error. If the loop count is greater than the configured maximum value (as a safeguard against inadvertently creating infinite message loops), or if a pack isn’t available for some other reason, you’ll get a nil pack and a non-nil error.

Once a pack has been obtained, a filter plugin can populate its Message struct using any of its provided mutator methods. (Note that this is the only time that it is safe to mutate a Message struct from within filter plugin code, since we know that this message has not yet hit the router and there is no risk of a race condition.) The pack can then be injected into the Heka message router queue, where it will be checked against all plugin message matchers, by passing it to the FilterRunner.Inject(pack *PipelinePack) method. Note that, again as a precaution against message looping, a plugin will not be allowed to inject a message which would get a positive response from that plugin’s own matcher.


In contrast to the Input plugin API, and older versions of the Filter plugin API, filter plugin code should not call the PipelinePacks’ Recycle method when a message has completed its processing. Message recycling is now handled by the FilterRunner.


Encoder plugins are the inverse of decoders. They convert Message structs into raw bytes that can be delivered to the outside world. Some encoders will serialize an entire Message struct, such as the Protobuf Encoder which uses Heka’s native protocol buffers format. Other encoders extract data from the message and insert it into a different format such as plain text or JSON.

The Encoder interface consists of one method:

type Encoder interface {
    Encode(pack *PipelinePack) (output []byte, err error)

This method accepts a PiplelinePack containing a populated message object and returns a byte slice containing the data that should be sent out, or an error if serialization fails for some reason. If the encoder wishes to swallow an input message without generating any output (such as for batching, or because the message contains no new data) then nil should be returned for both the output and the error.

Unlike the other plugin types, encoders don’t have a PluginRunner, nor do they run in their own goroutines. Outputs invoke encoders directly, by calling the Encode method exposed on the OutputRunner. This has the same signature as the Encoder interface’s Encode method, to which it will will delegate. If use_framing is set to true in the output’s configuration, however, the OutputRunner will prepend Heka’s Stream Framing to the generated binary data.

Outputs can also directly access their encoder instance by calling OutputRunner.Encoder(). Encoders themselves don’t handle the stream framing, however, so it is recommended that outputs use the OutputRunner method instead.

Even though encoders don’t run in their own goroutines, it is possible that they might need to perform some clean up at shutdown time. If this is so, the encoder can implement the NeedsStopping interface:

type NeedsStopping interface {

And the Stop method will be called during the shutdown sequence.


Finally we come to the output plugins, which are responsible for receiving Heka messages and using them to generate interactions with the outside world. Output plugins interact with Heka nearly identically to filter plugins.

Relevant Interfaces

There are three interfaces related to output plugin implementations. The first of these is the Output interface:

type Output interface {
    Prepare(or OutputRunner, h PluginHelper) (err error)

The only difference between this and the Filter interface is that the first argument to Prepare is and OutputRunner and not a FilterRunner. The two interfaces are used in the same way, with Prepare being called before message processing starts and CleanUp being called after message processing has completed.

The other two interfaces relevant to output plugins are MessageProcessor and TickerPlugin, which are used in precisely the same manner as they are with filter plugins, supporting the same special return errors.


Like filters, outputs can be configured to support disk buffering, and should call the OutputRunner’s UpdateCursor method as described here to advance the buffer cursor when appropriate.

Message Encoding

The primary way that outputs differ from filters is that output plugins need to either serialize or extract data from the messages they receive and then send that serialized or extracted data to an external destination. The serialization extraction should typically be performed by the output’s specified encoder plugin. The OutputRunner exposes the following methods to assist with this:

Encode(pack *PipelinePack) (output []byte, err error)
UsesFraming() bool
Encoder() (encoder Encoder)

The Encode method will use the specified encoder to convert the pack’s message to binary data, then if use_framing was set to true in the output’s configuration it will prepend Heka’s Stream Framing. The UsesFraming method will tell you whether or not use_framing was set to true. Finally, the Encoder method will return the actual encoder that was registered. This is useful to check to make sure that an encoder was actually registered, but generally you will want to use OutputRunner.Encode and not Encoder.Encode, since the latter will not honor the output’s use_framing specification.


In contrast to the Input plugin API, and older versions of the Output plugin API, output plugin code should not call the PipelinePacks’ Recycle method when a message has completed its processing. Message recycling is now handled by the OutputRunner.

Registering Your Plugin

The last step you have to take after implementing your plugin is to register it with Heka so it can actually be configured and used. You do this by calling the pipeline package’s RegisterPlugin function:

func RegisterPlugin(name string, factory func() interface{})

The name value should be a unique identifier for your plugin, and it should end in one of “Input”, “Splitter”, “Decoder”, “Filter”, “Encoder”, or “Output”, depending on the plugin type.

The factory value should be a function that returns an instance of your plugin, usually a pointer to a struct, where the pointer type implements the Plugin interface and the interface appropriate to its type (i.e. Input, Splitter, Decoder, etc).

This sounds more complicated than it is. Here are some examples from Heka itself:

RegisterPlugin("UdpInput", func() interface{} {return new(UdpInput)})
RegisterPlugin("TcpInput", func() interface{} {return new(TcpInput)})
RegisterPlugin("ProtobufDecoder", func() interface{} {return new(ProtobufDecoder)})
RegisterPlugin("CounterFilter", func() interface{} {return new(CounterFilter)})
RegisterPlugin("StatFilter", func() interface{} {return new(StatFilter)})
RegisterPlugin("LogOutput", func() interface{} {return new(LogOutput)})
RegisterPlugin("FileOutput", func() interface{} {return new(FileOutput)})

It is recommended that RegisterPlugin calls be put in your Go package’s init() function so that you can simply import your package when building hekad and the package’s plugins will be registered and available for use in your Heka config file. This is made a bit easier if you use plugin_loader.cmake, see Building hekad with External Plugins.

MessageProcessor Interface

Filter and output plugins should both implement the MessageProcessor interface:

type MessageProcessor interface {
    ProcessMessage(pack *PipelinePack) (err error)

Once initialization for a filter or an output plugin has been finalized, the ProcessMessage method will be called repeatedly, once for every message that is delivered to the plugin. The plugin should process the provided message as needed and should signal the results of the processing with the return value. If the message is successfully processed, nil should be returned. If the message cannot be processed and should be dropped, then an error should be returned; Heka will emit the error message to the console and continue.

Special MessageProcessor Return Errors

Heka provides a couple of special error types to pass additional information about the results of a ProcessMessage call. The first is a RetryMessageError. This is for when a message can’t be processed but future attempts may succeed, like when an output tries to write to an external service which is down. You can instantiate such an error using the pipeline.NewRetryMessageError function, defined as:

func NewRetryMessageError(msg string, subs ...interface{}) RetryMessageError

The variadic subs argument can be used to inject Printf style substitutions into the error’s message string.

When a RetryMessageError is returned, the next call to ProcessMessage will pass in the same PipelinePack, allowing the plugin to try again. If the error persists and ProcessMessage returns several RetryMessageErrors in a row, an exponential back-off time delay will happen between subsequent retries, to a maximum of one second.

The other special errors type is a PluginExitError, for cases where something went wrong during processing from which the plugin cannot recover. They are created using pipeline.NewPluginExitError:

func NewPluginExitError(msg string, subs ...interface{}) PluginExitError

Returning such an error will cause the plugin’s runner to exit its message processing loop entirely. If any restarting behavior has been configured, it will be applied after the exit.

TickerPlugin Interface

Most filter and output plugins will want to implement the TickerPlugin interface:

type TickerPlugin interface {
    TimerEvent() (err error)

Providing the TickerPlugin interface is optional for any filter or output plugin. If a plugin provides this interface, then the TimerEvent method will be called in regular intervals specified by the filter’s ticker_interval configuration setting. If TimerEvent is implemented but the ticker_interval option is set to zero, then TimerEvent will never be called. If a filter does not provide the TickerPlugin interface but the config specifies a ticker_interval, then the configuration will be considered to be invalid and Heka will not start.

If the TimerEvent method returns an error, that error will be logged to Heka’s console output. TimerEvent also supports returning the special PluginExitError type, in which case the plugin’s runner will exit exactly as if such an error were returned from the ProcessMessage method.

Note that the ProcessMessage and TimerEvent methods will never be called concurrently for the same plugin. This means that you do not need to worry about race conditions between these two methods. Race conditions are possible, of course, between either of these methods and any additional goroutines that may have been started by the Prepare method; it is up to the developer to manage mutable state carefully to avoid such conditions.

Updating Buffer Cursor

All filter and output plugins support optional use of disk buffering for all messages delivered to the plugin. Each buffer maintains a cursor into the buffer’s stream, which should be advanced past a given record when that record has been completely processed.

At first it might seem that the cursor should be updated automatically every time ProcessMessage is called and no error is returned. This doesn’t work, however, because plugins will sometimes process in batches, and the cursor shouldn’t be advanced past any of the messages in the batch until the entire batch is finalized.

For this reason plugins must manually update the queue cursor. Every message has a unique, opaque cursor string value, stored on the PipelinePack as the QueueCursor attribute. When a plugin has successfully finalized processing of a message, it should notify Heka by passing this QueueCursor value back to the runner’s UpdateCursor method, like so:


Where runner is the FilterRunner or OutputRunner, depending on your plugin type.

When messages are being processed in batches, after each batch UpdateCursor should be called once, with the QueueCursor value from the last message in the batch.

Each UpdateCursor call should pass in a cursor for a message that came later in the stream than the previous UpdateCursor call. In other words, it is not possible to move the cursor backwards; if UpdateCursor is passed a cursor value for a messages that was earlier in the stream, an error will be logged to stderr and the cursor will not be updated.