[jira] [Commented] (SOLR-10233) Add support for different replica types in Solr

["Tomás Fernández Löbbe (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SOLR-10233?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16001357#comment-16001357 ] 

Tomás Fernández Löbbe commented on SOLR-10233:
----------------------------------------------

I’ve been doing some work on improving the tests in the branch. Also, now RTG requests with distrib=true will avoid PASSIVE and non-leader APPEND replicas. For distrib=false cases, PASSIVE replicas will error, but APPEND will proceed the same as today, since this is needed for leader sync.

Regarding the naming, here are the options proposed
* REALTIME/PUSH/NRT/WRITER/INDEXER
* APPEND/SYNC/TLOG/HOTCOPY/LAZY
* PASSIVE/PULL/INDEX/COPY/SLAVE/READ-ONLY

Based on the suggestions and comments, I’m going to rename:
* REALTIME -> NRT: It makes it obvious that if you want NRT results you need these kinds of replicas. Plus we’ve been saying that Solr provides search in NRT and this is the mode currently supported
* APPEND -> I’m between LAZY and TLOG. The first one is more high level, the name itself doesn’t say much of how the internals work. It’s less specific than TLOG, but it does give you the idea that it will fall behind. Also, I think it kind of makes sense because the replica will “apply the updates to the index only if required (it becomes leader)”. LAZY may be easily confused with PASSIVE. TLOG is good because it’s clearer than LAZY in what the replica does. The main issue I see is that the REALTIME (or NRT) also has a transaction log, so may be confusing. Since Mark proposed TLOG, I’m guessing he is +1 on that one so I’ll just go with it, since i’m on the fence. 
* PASSIVE -> PULL, It is more obvious than “PASSIVE” on how the replica will work, and it doesn’t get confused with a replica state.

Feel free to comment if you have some good reason against those names. 

I’ll be doing some more work on the tests this week. My plan is to commit this to master soon to have it for 7.0. Placement strategy and client changes are going to be done as followup tasks, I don’t think those should block this.

> Add support for different replica types in Solr
> -----------------------------------------------
>
>                 Key: SOLR-10233
>                 URL: https://issues.apache.org/jira/browse/SOLR-10233
>             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
>         Attachments: 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_ replicas.
> 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":"http://127.0.0.1:8983/solr",
>           "node_name":"127.0.0.1:8983_solr",
>           "state":"active",
>           "leader":"true",
>           **"type": "realtime"**},
>         "core_node10":{
>           "core":"gettingstarted_shard1_replica2",
>           "base_url":"http://127.0.0.1:7574/solr",
>           "node_name":"127.0.0.1:7574_solr",
>           "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 continue. 
> 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_ replicas
> 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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