Bilal Waheed created SOLR-14339:
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Summary: SHARED replica's freshness
Key: SOLR-14339
URL: https://issues.apache.org/jira/browse/SOLR-14339
Project: Solr
Issue Type: Sub-task
Components: SolrCloud, SolrJ
Reporter: Bilal Waheed
*Problem*
The source of truth for a SHARED replica lives in the shared store. At any give
point in time, locally a SHARED replica can be missing some or all of its
shard's indexing. The amount of indexing that is missing from a SHARED replica
is arbitrary. This is even true for a leader replica because we do not update
SHARED replica from shared store at startup/recovery/leader-election etc. All
this means that a query served by SHARED replica can be blind to arbitrary
amount of indexing.
There are few requirements we would like to support.
# Query visibility into whatever have been indexed so far is not stale beyond
some well defined SLA.
# A client can issue a Solr query that has complete visibility into whatever
has been indexed so far. This is primarily needed for tests.
# Solr query results has information about the freshness of index used to
serve the query.
# Scalability: We would like to eventually support high number (~10K) of
shared store based collections per Solr cluster. With understanding that a
large majority (~80%) will not be used most of the time. The ones that will be
used will also go through some daily usage cycle (less usage at night time). In
other words, we do not want to proactively refresh SHARED replica from shared
store until unless it is needed for querying or indexing.
For indexing we already have things covered where we synchronously ensures that
the leader replica is up to date with shared store before proceeding with new
indexing. But we need a solution for queries.
*Proposal*
Two duration configuration values called *freshnessTolerance* and
*refreshInterval* are introduced. Both are defined in units of seconds. Each
SHARED replica tracks (in JVM memory) the last refresh time (LRT) it was
brought to be up to date with the index data in the shared store (last time it
read metadataSuffix^[1]^ from Zookeeper to evaluate its freshness, followed by
a pull from shared store when needed). The time immediately before reading the
metadataSuffix becomes the LRT. If there is nothing to pull then we simply
update replica’s LRT. But if there is something to pull then LRT is updated
only after the pull is successfully finished. Leader will also update its LRT
after successful indexing batch. In successful indexing case the time just
before the update of metadataSuffix in ZK becomes the LRT.
A replica with LRT less than equal to *freshnessTolerance* duration ago (when
compared to query arrival time) is allowed to serve a query. But if the LRT is
more than freshnessTolerance duration ago then we will check if there is any
new indexing to pull or not. If there is none we will update the LRT of the
replica and serve the query. If there is new indexing to pull then we will
enqueue a pull from shared store and fail the query immediately by throwing
"*Replica is not fresh enough to serve the query*" error.
SolrCloud already has a logic where if a replica is failed to serve the query
then it will try the other available replicas of the shard. If none of the
replicas of a shard are fresh enough to serve the query then Solr will return
*NoReplicaIsFreshEnough* error.
The *refreshInterval* config denotes a shorter duration than freshnessTolerance
config. A replica getting a continuous stream of queries will at least refresh
itself every refreshInterval duration.
Clients will have an option to *override freshnessTolerance on per query basis*
i.e. it can be passed as a parameter to query. For immediate readability (serve
the query with whatever has been indexed so far) client will pass 0 for
freshnessTolerance.
If a replica serves the query it will return the *timeSinceLastRefresh*
duration of the replica (in milliseconds) in query response. If in query fanout
there are multiple shards(replicas) involved then the timeSinceLastRefresh
duration of the query will be maximum of timeSinceLastRefresh of all the
replicas that contributed to the query. Because of clock skewness issues
between client and Solr, timestamp of the last refresh does not carry much
meaning therefore we are returning the length of time since the last refresh.
[1] *metadataSuffix*: One collection can be made up of multiple shards and each
shard can have multiple replicas. For each shard only one replica will be
acting as a leader and taking indexing and pushing to shared store. Failures
can happen and we can have old leader and new leader working at the same time.
To keep index correct we write a unique string in zookeeper at the end of each
successful indexing batch. We call that unique string metadataSuffix and it is
a per-shard metadata. It is not important to understand how it is used but that
it exists and for each shard this is the source of truth for last successful
indexing batch. Each replica has a JVM cache telling on what metadataSuffix
version of the index it is at.
*Pseudo code*
{code:java}
// A SolrQueryRequest q has arrived for replica r of shard s.
// Use of query arrival time is conservative. If we somehow has the ability
// to subtract network time from this, we will still be correct in our logic.
long queryArrivalTime = q.getStartTime();
long lastRefreshTime = getLastRefreshTime(r);
Long freshnessToleranceParameter = getFreshnessToleranceParameter(q);
long acceptableFreshnessTolerance = freshnessToleranceParameter != null ?
freshnessToleranceParameter : getFreshnessTolerance();
acceptableFreshnessTolerance = TimeUnit.MILLISECONDS.convert(
acceptableFreshnessTolerance, TimeUnit.SECONDS);
long minimumAcceptableLastRefreshTime = queryArrivalTime -
acceptableFreshnessTolerance;
if (lastRefreshTime >= minimumAcceptableLastRefreshTime) {
long refreshInterval = TimeUnit.MILLISECONDS.convert(getRefreshInterval(),
TimeUnit.SECONDS);
if ((lastRefreshTime + refreshInterval) < System.currentTimeMillis()) {
enqueuePullFromSharedStore(r);
}
} else {
long newLastRefreshTime = System.currentTimeMillis();
String metadataSuffixInCache = getMetadataSuffixFromCache(r);
String metadataSuffixInZK = getMetadataSuffixFromZK(s);
if (metadataSuffixInCache.equals(metadataSuffixInZK)) {
// No new indexing since we last pulled from shared store.
// we can just update our last refresh time and we will be fresh enough to
// serve the query
updateLastRefreshTime(r, newLastRefreshTime);
} else {
enqueuePullFromSharedStore(r);
throw new SolrException(ErrorCode.SERVER_ERROR, "Replica is not fresh
enough to serve the query");
}
}
// serve the query...
{code}
*Minimizing NoReplicaIsFreshEnough errors*
Some steady state scenarios this proposal will cover without failing the query
with NoReplicaIsFreshEnough error.
* A replica getting continuous query stream will keep serving them.
* Query stream is not continuous and there is no query activity for
freshnessTolerance duration.
** If there is no new indexing since the last query then any replica that has
already been queried before will be able to server the query.
** If there is new indexing then only the leader replica will be able to
server the query (assuming leader has not failed after indexing)
We will still have the scenarios where we will run into NoReplicaIsFreshEnough
error.
* Cold start, shard was not active (no indexing, no querying).
* Deployments doing a rolling restart of all the replicas.
* There was no query activity for freshnessTolerance duration of time and then
there was some indexing. After indexing leader died.
Minimizing NoReplicaIsFreshEnough errors is out of the scope for this Jira.
There will be a separate Jira on that.
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