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From "ASF subversion and git services (JIRA)" <>
Subject [jira] [Commented] (SOLR-10233) Add support for different replica types in Solr
Date Wed, 07 Jun 2017 00:49:18 GMT


ASF subversion and git services commented on SOLR-10233:

Commit 97655b880c0230c0d42baba314c28831ee729323 in lucene-solr's branch refs/heads/master
from [~tomasflobbe]
[;h=97655b8 ]

SOLR-10233: Cleanup warnings from ReplicateFromLeader

> Add support for different replica types in Solr
> -----------------------------------------------
>                 Key: SOLR-10233
>                 URL:
>             Project: Solr
>          Issue Type: New Feature
>      Security Level: Public(Default Security Level. Issues are Public) 
>          Components: SolrCloud
>            Reporter: Tomás Fernández Löbbe
>            Assignee: Tomás Fernández Löbbe
>             Fix For: master (7.0)
>         Attachments: 11431.consoleText.txt, SOLR-10233.patch, SOLR-10233.patch, SOLR-10233.patch,
SOLR-10233.patch, SOLR-10233.patch
> For the majority of the cases, current SolrCloud's  distributed indexing is great. There
is a subset of use cases for which the legacy Master/Slave replication may fit better:
> * Don’t require NRT
> * LIR can become an issue, prefer availability of reads vs consistency or NRT
> * High number of searches (requiring many search nodes)
> SOLR-9835 is adding replicas that don’t do indexing, just update their transaction
log. This Jira is to extend that idea and provide the following replica types:
> * *Realtime:* Writes updates to transaction log and indexes locally. Replicas of type
“realtime” support NRT (soft commits) and RTG. Any _realtime_ replica can become a leader.
This is the only type supported in SolrCloud at this time and will be the default.
> * *Append:* Writes to transaction log, but not to index, uses replication. Any _append_
replica can become leader (by first applying all local transaction log elements). If a replica
is of type _append_ but is also the leader, it will behave as a _realtime_. This is exactly
what SOLR-9835 is proposing (non-live replicas)
> * *Passive:* Doesn’t index or writes to transaction log. Just replicates from _realtime_
or _append_ replicas. Passive replicas can’t become shard leaders (i.e., if there are only
passive replicas in the collection at some point, updates will fail same as if there is no
leaders, queries continue to work), so they don’t even participate in elections.
> When the leader replica of the shard receives an update, it will distribute it to all
_realtime_ and _append_ replicas, the same as it does today. It won't distribute to _passive_
> By using a combination of _append_ and _passive_ replicas, one can achieve an equivalent
of the legacy Master/Slave architecture in SolrCloud mode with most of its benefits, including
high availability of writes. 
> h2. API (v1 style)
> {{/admin/collections?action=CREATE…&*realtimeReplicas=X&appendReplicas=Y&passiveReplicas=Z*}}
> {{/admin/collections?action=ADDREPLICA…&*type=\[realtime/append/passive\]*}}
> * “replicationFactor=” will translate to “realtime=“ for back compatibility
> * if _passive_ > 0, _append_ or _realtime_ need to be >= 1 (can’t be all passives)
> h2. Placement Strategies
> By using replica placement rules, one should be able to dedicate nodes to search-only
and write-only workloads. For example:
> {code}
> shard:*,replica:*,type:passive,fleet:slaves
> {code}
> where “type” is a new condition supported by the rule engine, and “fleet:slaves”
is a regular tag. Note that rules are only applied when the replicas are created, so a later
change in tags won't affect existing replicas. Also, rules are per collection, so each collection
could contain it's own different rules.
> Note that on the server side Solr also needs to know how to distribute the shard requests
(maybe ShardHandler?) if we want to hit only a subset of replicas (i.e. *passive *replicas
only, or similar rules)
> h2. SolrJ
> SolrCloud client could be smart to prefer _passive_ replicas for search requests when
available (and if configured to do so). _Passive_ replicas can’t respond RTG requests, so
those should go to _realtime_ replicas. 
> h2. Cluster/Collection state
> {code}
> {"gettingstarted":{
>   "replicationFactor":"1",
>   "router":{"name":"compositeId"},
>   "maxShardsPerNode":"2",
>   "autoAddReplicas":"false",
>   "shards":{
>     "shard1":{
>       "range":"80000000-ffffffff",
>       "state":"active",
>       "replicas":{
>         "core_node5":{
>           "core":"gettingstarted_shard1_replica1",
>           "base_url":"",
>           "node_name":"",
>           "state":"active",
>           "leader":"true",
>           **"type": "realtime"**},
>         "core_node10":{
>           "core":"gettingstarted_shard1_replica2",
>           "base_url":"",
>           "node_name":"",
>           "state":"active",
>           **"type": "passive"**}},
>       }},
>     "shard2":{
>       ...
> {code}
> h2. Back compatibility
> We should be able to support back compatibility by assuming replicas without a “type”
property are _realtime_ replicas. 
> h2. Failure Scenarios for passive replicas
> h3. Replica-Leader partition
> In SolrCloud today, in this scenario the replica would be placed in LIR. With _passive_
replicas, replicas may not be able to replicate from some time (and fall behind with the index)
but queries can still be served. Once the connection is re-established the replication will
> h3. Replica ZooKeeper partition
> _Passive_ replica will leave the cluster. “Smart clients” and other replicas (e.g.
for distributed search) won’t find it and won’t query on it. Direct search requests to
the replica may still succeed. 
> h3. Passive replica dies (or is unreachable)
> Replica won’t be query-able. On restart, replica will recover from the leader, following
the same flow as _realtime_ replicas: set state to DOWN, then RECOVERING, and finally ACTIVE.
_Passive_ replicas will use a different {{RecoveryStrategy}} implementation, that omits *preparerecovery,*
and peer sync attempt, it will jump to replication . If the leader didn't change, or if the
other replicas are of type “append”, replication should be incremental. Once the first
replication is done, passive replica will declare itself active and start serving traffic.
> h3. Leader dies
> Passive replica won’t be able to replicate. The cluster won’t take updates until
a new leader is elected. Once a new leader is elected, updates will be back to normal. Passive
replicas will remain active and serving query traffic during the “write outage”. Once
the new leader is elected the replication will restart (maybe from a different node)
> h3. Leader ZooKeeper partition
> Same as today. Leader will abandon leadership and a new replica will be elected as leader.
> h2. Q&A
> h3. Can I use a combination of _passive_ + _realtime_?
> You could. The problem is that, since _realtime_ generate their own index, any change
of leadership could trigger a full replication from all the _passive_ replicas. The biggest
benefits of _append_ replicas is that they share the same index files, which means that even
if the leader changes, the number of segments to replicate will remain low. For that reason,
using _append_ replicas is recommended when using _passive_.
> h3. Can I use _passive_ + _append_ + _realtime_?
> The issue with mixing _realtime_ replicas with _append_ replicas is that if a different
_realtime_ replica becomes the leader, the whole purpose of using _append_ replicas is defeated,
since they will all have to replicate the full index. 
> h3. What happens if replication from *passives* fail?
> TBD: In general we want those replicas to continue serving search traffic, but we may
want to have a way to say “If can’t replicate after X hours put yourself in recovery”
or something similar.
> [~varunthacker] suggested that we include in the response time since the last successful
replication, and then the client can choose what to do with the results (in a multi-shard
request, this date would be the oldest of all shards).
> h3. Do _passive_ replicas need to replicate from the leader only?
> This is not necessary. _Passive_ replicas can replicate from any _realtime_ or _append_
replicas, although this would add some extra waiting time for the last updates. Replicating
from a _realtime_ replica may not be a good idea, see the question “Can I use a combination
of _passive_ + _realtime_?”
> h3. What if I need NRT?
> Then you can’t query _append_ or _passive_ replicas. You should use all _realtime_
> h3. Will new _passive_ replicas start receiving traffic immediately after added?
> _passive_ replicas will have the same states as _realtime_/_append_ replicas, they’ll
join the cluster as “DOWN” and be moved to “RECOVERY” until they can replicate from
the leader. Then they’ll start the replication process and become “ACTIVE”, at this
point they’ll start responding queries. They'll use a different {{RecoveryStrategy}} that
skips peer sync and buffering of docs, and just replicates.
> h3. What if a _passive_ replica receives an update?
> This will work the same as today with non-leader replicas, it will just forward the update
to the correct leader.
> h3. What is the difference between using active + passive with legacy master/slave?
> These are just some I can think of:
> * You now need ZooKeeper to run in SolrCloud mode
> * High availability for writes, as long as you have more than 1 active replica
> * Shard management by Solr at index time and query time.
> * Full support for Collections and Collections API
> * SolrCloudClient support
> I'd like to get some thoughts on this proposal.

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