go-docappender provides a Go API for append-only Elasticsearch document indexing.

This software is licensed under the Apache 2 license.

go-docappender is an evolution of the Elastic APM Server Elasticsearch output, and was formerly known as modelindexer.

Prior to 8.0, APM Server used the libbeat Elasticsearch output. 8.0 introduced a new output called "modelindexer", which was coupled to the APM Server event data model and optimised for APM Server's usage. From 8.0 until 8.5, modelindexer processed events synchronously and used mutexes for synchronized writes to the cache. This worked well, but didn't seem to scale well on bigger instances with more CPUs.

flowchart LR;
    subgraph Goroutine
        Flush;
    end
    AgentA & AgentB-->Handler;
    subgraph Intake
    Handler<-->|semaphore|Decode
    Decode-->Batch;
    end
    subgraph ModelIndexer
    Available-.->Active;
    Batch-->Active;
    Active<-->|mutex|Cache;
    end
    Cache-->|FullOrTimer|Flush;

    Flush-->|bulk|ES[(Elasticsearch)];
    Flush-->|done|Available;

In APM Server 8.6.0, modelindexer was redesigned to accept events asynchronously, and run one or more "active indexers", which would each pull events from an in-memory queue and (by default) compress them and write them to a buffer. This approach reduced lock contention, and allowed for automatically scaling the number of active indexers up and down based on queue utilisation, with an upper bound based on the available memory.

flowchart LR;
    subgraph Goroutine11
        Flush1(Flush);
    end
    subgraph Goroutine22
        Flush2(Flush);
    end
    AgentA & AgentB-->Handler;
    subgraph Intake
    Handler<-->|semaphore|Decode
    Decode-->Batch;
    end
    subgraph ModelIndexer
    Batch-->Buffer;
    Available;
        subgraph Goroutine1
            Active1(Active);
            Active1(Active)<-->Cache1(Cache);
            Cache1(Cache)-->|FullOrTimer|Flush1(Flush);
        end
        subgraph Goroutine2
            Active2(Active);
            Active2(Active)<-->Cache2(Cache);
            Cache2(Cache)-->|FullOrTimer|Flush2(Flush);
        end
        subgraph Channel
            Buffer-->Active1(Active) & Active2(Active);
        end
        Available-.->Active1(Active) & Active2(Active);
    end

    Flush1(Flush) & Flush2(Flush)-->|bulk|ES[(Elasticsearch)];
    Flush1(Flush) & Flush2(Flush)-->|done|Available;

We use GitHub releases to manage tagged releases, and aim to conform to semver in our release naming.

To create a new release, use the new release interface, and use GitHub's generate release notes to get an automatically-generated list of changes made since the last release.