Replication Algorithm

Benoit Chesneau edited this page Jun 20, 2013 · 4 revisions

The replication algorithm in pseudo code.


The replication algorithm in details

Pseudo-code extracted from the couch_replicator application in rcouch.

###0. start

Parse the replicator document Calculate a replication id for this replication task

###1. init

(when the replication task is created)

Get source infos (/source) Get target info
SourceSeq = update_seq in source infos

find replication logs:
    logs are stored in checkpoints document
    checkpoint document id is defined by `logid$

    logid = _local/replication id
    get replications logs on [Source, Target]

compare replication logs between Source and target:

    if Source.`sessionid` == Target.`sessionid`:
        """ records have the same session id, then we have a valid
        replication history """

        StartSeq = Source.`source_last_seq` or 0
        History = Source.`history`

        """ scanning histories to find differences """
        compare replication history between Source and Target:

            An history is a list of committed replications.
            we iterate over each elements side by side:

                while source != [] and target != []:
                    S0, SourceRest = source pop left
                    T0, TargetRest = target pop left

                    if Target has S0.`sessionid`:
                        StartSeq = S0.`recorded_seq` or 0
                        History = SourceRest

                        if SourceRest has Target.`sessionid`:
                            StartSeq = T0.`recorded_seq` or 0
                            History = TargetRest

                    source = SourceRest
                    target = TargetRest

                if not StartSeq:
                    """ no common ancestors found """

                    StartSeq = 0
                    History = []

    return StartSeq, History

note: Does the replicator try to find old checkpoint documents by testing the old document id is, then changing to new id?

###2. read changes on source since StartSeq

GET /_changes or subscribe to local changes
    style: 'all_docs'
    feed : 'normal' or 'continuous' if continuous=true

    if filter:
        pass filter params

note: when reading changes make sure the doc we receive has an id. if not ignore it. (some versions of couchdb had a bug)

###3. on change

find missing on target: get all pair {Id, Revs} and revscount:
        idrevs = []
        revscount = 0
        for doc in changes:
            idrevs += [{Id, Revs}]
            revscount += len(Revs)

    get missing revs on target with idrevs:
        if target is remote:
            convert revs to rev str
            body = {id:revsttr, ...}

            POST /sourcedb/_revs_diff jsonbody
            get local missing revs

        return list of {Id, MissingRevs, PossibleAncestors} as idsrevs

    get missing count
        missingcount = 0
        for {Id, MissingRevs, PossibleAncestors} in missingrevs:
            missingcount += len(MissingRevs)

    stats = {revscount, missingcount}
    return {idsrevs, stats}

###4. process changes:

if source is local: for {Id, MissingRevs, PossibleAncestors} in idsrevs: Options = {atts_since, PossibleAncestors}, latest] fetch doc: Doc = open_doc_revs(db, Id, MissingRevs, Options)
        maybe flush doc:
            # update doc use new_edits=alse function to reuse the same
            # revision and not edit the documen
            updatd doc if nb doc <= worker_batch_size and still changes
            to process
                else wait next changes
    spawn a process to fetch a doc
    Use multipart API:

    ---- boundary, name=doc
    ---- boundary, name=attname

    once read ->
        maybe flush doc

###5. update state

Once the documents are stored on the target, the state is updated. The checkpoint will be updated with this state at the given interval

###6. listen on other changes

If the replication is continuous, wait for other changes.

How to calculate a replication ID?

There have been 3 versions of the CouchDB replication protocol that can be detcted using the replicaton ID.

The current version of the replication ID is calculated following such rules:

base = [node uuid, src uri, target uri]
if property "filter"
    if filter starts with "_" // _view for ex
         base = base + [filter name, query_params]
        fetch code of the filter function
        base = base + [code, query_params)

repid = hex(md5(base))

Node UUID; string generated on first couchdb start and stored on the uuid key of the [couchdb] section in local.ini. Actually can be anything that identify the node uniquely.

Changes of replication ID over protocols

protocol version 2:

    base = [Host, Port, Source, Target]

protocol version 1:

    base = [Host, Source, Target]


Internally the full replication Id is a tupple collecting specific options for this replication:

extra = []
if continuous option:
    extra += [continuous]
if create_target option:
    extra += [create_target]

{replication ID, extra}

Store a checkpoint document

A checkpoint document is a local document stored both on source and target on a given interval. This interval is actually fixed in source of CouchDB at 5000ms.

The checkpoint document keeps current state of the replication and allows us to start the replication from the last point. A checkpoint document keeps the last 50 sessions to find the differences between the source and target.

The checkpoint document ID is _local/replicationID and is unique for a replication.

History entry properties:

  • session_id
  • start_time
  • end_time
  • start_last_seq
  • end_last_seq
  • recorded_seq
  • missing_checked
  • missing_found
  • docs_read
  • docs_written
  • doc_write_failure

Checkpoint document

    "session_id": "string",
    "source_last_seq": "last source seq",
    "replication_id_version": 3,
    "history": [ NewHistoryEntry, ... last 49 entries ]


Deprecated and only here to support the version 2 of the replication protocol, the following properties are added:

    "start_time": "ISO-8601"
    "end_time": "ISO-8601"
    "docs_read": 100
    "docs_written": 100
    "doc_write_failure": 0

CouchDB implementation

Once a replication task is posted or a replication document is created the couch_replicator process is created. This process is responsible of keeping the replication state updated: it receives the last source sequence updated and update the checkpoint document. This process is also watching the source and target to detect if they are still existing and if a compaction start.

The couch_replicator is also responsible of launching such processes:

  • start changes reader process : It adds the changes from the source db to the ChangesQueue.
  • start changes manager: responsible for dequeing batches from the changes queue and deliver them to the worker processes.
  • spawn replicator workers: They ask the changes queue manager for a a batch of _changes rows to process -> check which revs are missing in the target, and for the missing ones, it copies them from the source to the target.

For each replication tasks a pool of connection is created.

Replication doc main properties:

  • source
  • target

source or target can be:

  • dbname string if local

  • db URI string if remote. It can contain the creadentials. Ex:


  • object:

    url: db URI string headers: hash of {headername, headervalue} K/V pairs auth: oauth: oauth object consumer key token token_secret consumer_secret signature_method (default HMAC_SHA1): HMAC_SHA1, PLAINTEXT, RSA-SHA1

replication options

replication_id (when created)
proxy: proxy url


Default options:

worker_processes = 4
woerker_batch_size = 500
htttp_connections = 20
connection_timeout = 30000
retries_per_request = 4

SSL options only set in the settings

if ssl_trusted_certificates_file
    no check
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