wohali commented on a change in pull request #123: Documentation for the
scheduling replicator
URL:
https://github.com/apache/couchdb-documentation/pull/123#discussion_r114016922
##########
File path: src/replication/replicator.rst
##########
@@ -154,98 +203,253 @@ the following order:
.. code-block:: javascript
{
- "_id": "doc_A",
- "source": "http://myserver.com:5984/foo";,
- "target": "http://user:pass@localhost:5984/bar";
+ "_id": "my_rep",
+ "source": "http://user:pass@localhost:5984/foo";,
+ "target": "http://user:pass@localhost:5984/bar";,
+ "create_target": true,
+ "continuous": true
}
and
.. code-block:: javascript
{
- "_id": "doc_B",
- "source": "http://myserver.com:5984/foo";,
- "target": "http://user:pass@localhost:5984/bar";
- }
-
-Both describe exactly the same replication (only their ``_ids`` differ). In
this
-case document ``doc_A`` triggers the replication, getting updated by CouchDB
-with the fields ``_replication_state``, ``_replication_state_time`` and
-``_replication_id``, just like it was described before. Document ``doc_B``
-however, is only updated with one field, the ``_replication_id`` so it will
-look like this:
-
-.. code-block:: javascript
-
- {
- "_id": "doc_B",
- "source": "http://myserver.com:5984/foo";,
+ "_id": "my_rep_dup",
+ "source": "http://user:pass@localhost:5984/foo";,
"target": "http://user:pass@localhost:5984/bar";,
- "_replication_id": "c0ebe9256695ff083347cbf95f93e280"
+ "create_target": true,
+ "continuous": true
}
-While document ``doc_A`` will look like this:
-
-.. code-block:: javascript
-
- {
- "_id": "doc_A",
- "source": "http://myserver.com:5984/foo";,
- "target": "http://user:pass@localhost:5984/bar";,
- "_replication_id": "c0ebe9256695ff083347cbf95f93e280",
- "_replication_state": "triggered",
- "_replication_state_time": "2011-02-17T20:22:02+01:00"
- }
+Both describe exactly the same replication (only their ``_ids``
+differ). In this case document ``my_rep`` triggers the
+replication. While ``my_rep_dup``` will fail. Inspecting
+``_scheduler/docs`` explains exactly why it failed:
+
+.. code-block:: json
+
+ {
+ "database": "_replicator",
+ "doc_id": "my_rep_dup",
+ "error_count": 1,
+ "id": null,
+ "info": "Replication
`a81a78e822837e66df423d54279c15fe+continuous+create_target` specified by
document `my_rep_dup` already started, triggered by document `my_rep` from db
`_replicator`",
+ "last_updated": "2017-04-05T21:41:51Z",
+ "source": "http://adm:*****@localhost:5984/foo/";,
+ "start_time": "2017-04-05T21:41:51Z",
+ "state": "failed",
+ "target": "http://adm:*****@localhost:5984/bar/";
+ }
-Note that both document get exactly the same value for the ``_replication_id``
-field. This way you can identify which documents refer to the same replication
-
-you can for example define a view which maps replication IDs to document IDs.
+Notice the state for this replication is ``failed``. Unlike
+``crashing``, ``failed`` state is terminal. As long as both documents
+are present replicator will not retry to run my_rep_dub
+replication. Another reason could be malformed documents. For
+example if worker process count is specified as a string
+(``"worker_processes": "a few"``) instead of an integer.
+
+Replication Scheduler
+=====================
+
+Once replication jobs are created they are managed by the scheduler.
+The scheduler is the replication component which periodically stops
+some jobs and starts others. This behavior makes it posssible to have a
+larger number of jobs than what the cluster could run at one time.
+Replication jobs which keep failing will be penalized and forced to
+wait. The wait time increases exponentially with each consecutive
+failure.
+
+When deciding which jobs to stop and which to start, the scheduler
+uses a round-robin algorithm to ensure fairness. Jobs which have been
+running the longest time will be stopped, and jobs which have been
+waiting the longest time will be started.
+
+.. note:: Non-continuous (normal) replication are treated differently
+ once they start running. See :ref:`Normal vs Continuous
+ Replications` section for more information.
+
+The behavior of the scheduler can configured via ``max_jobs``,
+``interval`` and ``max_churn`` options. See :ref:`Replicator
+configuration section <config/replicator>` for additional information.
+
+Replication states
+==================
+
+Replication jobs during their life-cycle pass through various
+states. This is a diagram of all the states and transitions
+between them:
+
+.. figure:: ../../images/replication-state-diagram.svg
+ :align: center
+ :alt: Replication state diagram
+
+ Replication state diagram
+
+Green and red shapes represent replication job states.
Review comment:
Recent figures suggest that approximately 10% of men have red-green colour
blindness. Those people won't be able to tell the difference between your
healthy and unhealthy states, especially since they differ only in colour.
Can we change the colours used from green/red/yellow to an alternative
palette? Alternatively, might it make sense for unhealthy states to have a
slightly different shape?
Here are 2 excellent resources I've used in the past to help with this
problem:
* http://www.visibone.com/colorblind/
* http://safecolours.rigdenage.com/
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