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【Elasticsearch源码】 GET分析

带着疑问学源码,第四篇:Elasticsearch GET
代码分析基于:https://github.com/jiankunking/elasticsearch
Elasticsearch 7.10.2+

通过前3篇的学习,可以稍微总结一下Elasticsearch:

  • ES是一个集群,所以每个Node都需要和其他的Nodes 进行交互,这些交互是通过NodeClient来完成。
  • ES中RPC、HTTP请求都是基于Netty自行封装的:
  • Transport*Action 是比较核心的类集合:
    • Action -> Transport*Action
    • TransportAction -> TransportHandler(即使是本地Node也会通过发请求的方式,将处理转发到TransportHandler处理)
    • 真实干活的Transport*Action类(或者其父类)中doExecute(……)

目的

在看源码之前先梳理一下,自己对于GET流程疑惑的点:

  • 是不是根据Document _id通过hash找到对应的Shard?
  • 根据Document _id查询如何做到实时可见的?

源码分析

第二部分是代码分析的过程,不想看的朋友可以跳过直接看第三部分总结。

通过搜索/{index}/_doc/{id}可以找到RestGetAction,找到RestGetAction再加上前面的总结,其实就知道真实干活的是TransportGetAction

在TransportGetAction的父类TransportSingleShardAction中找到了doExecute:

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@Override
protected void doExecute(Task task, Request request, ActionListener<Response> listener) {
new AsyncSingleAction(request, listener).start();
}

// TransportSingleShardAction的AsyncSingleAction中
private AsyncSingleAction(Request request, ActionListener<Response> listener) {
this.listener = listener;

ClusterState clusterState = clusterService.state();
if (logger.isTraceEnabled()) {
logger.trace("executing [{}] based on cluster state version [{}]", request, clusterState.version());
}

// 集群nodes列表
nodes = clusterState.nodes();
ClusterBlockException blockException = checkGlobalBlock(clusterState);
if (blockException != null) {
throw blockException;
}

String concreteSingleIndex;
if (resolveIndex(request)) {
concreteSingleIndex = indexNameExpressionResolver.concreteSingleIndex(clusterState, request).getName();
} else {
concreteSingleIndex = request.index();
}
this.internalRequest = new InternalRequest(request, concreteSingleIndex);

// TransportGetAction中resolveRequest
// 解析请求,更新指定routing
resolveRequest(clusterState, internalRequest);

blockException = checkRequestBlock(clusterState, internalRequest);
if (blockException != null) {
throw blockException;
}

// 根据路由算法获取目标shard的迭代器或者根据优先级获选择目标节点
this.shardIt = shards(clusterState, internalRequest);
}


// TransportGetAction中
@Override
protected ShardIterator shards(ClusterState state, InternalRequest request) {
return clusterService.operationRouting()
.getShards(clusterService.state(), request.concreteIndex(), request.request().id(), request.request().routing(),
request.request().preference());
}

下面看一下OperationRouting中的getShards(……)看一下是如何获取到具体的shardId的:

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 public ShardIterator getShards(ClusterState clusterState, String index, String id, @Nullable String routing,
@Nullable String preference) {
return preferenceActiveShardIterator(shards(clusterState, index, id, routing), clusterState.nodes().getLocalNodeId(),
clusterState.nodes(), preference, null, null);
}

protected IndexShardRoutingTable shards(ClusterState clusterState, String index, String id, String routing) {
int shardId = generateShardId(indexMetadata(clusterState, index), id, routing);
return clusterState.getRoutingTable().shardRoutingTable(index, shardId);
}

public static int generateShardId(IndexMetadata indexMetadata, @Nullable String id, @Nullable String routing) {
final String effectiveRouting;
final int partitionOffset;

// routing参数解析可以参考具体的文档
// https://www.elastic.co/guide/en/elasticsearch/reference/current/docs-get.html
if (routing == null) {
assert(indexMetadata.isRoutingPartitionedIndex() == false) : "A routing value is required for gets from a partitioned index";
effectiveRouting = id;
} else {
effectiveRouting = routing;
}

if (indexMetadata.isRoutingPartitionedIndex()) {
partitionOffset = Math.floorMod(Murmur3HashFunction.hash(id), indexMetadata.getRoutingPartitionSize());
} else {
// we would have still got 0 above but this check just saves us an unnecessary hash calculation
partitionOffset = 0;
}

return calculateScaledShardId(indexMetadata, effectiveRouting, partitionOffset);
}

private static int calculateScaledShardId(IndexMetadata indexMetadata, String effectiveRouting, int partitionOffset) {
final int hash = Murmur3HashFunction.hash(effectiveRouting) + partitionOffset;

// we don't use IMD#getNumberOfShards since the index might have been shrunk such that we need to use the size
// of original index to hash documents
return Math.floorMod(hash, indexMetadata.getRoutingNumShards()) / indexMetadata.getRoutingFactor();
}

到这里可以知道ES就是通过Document _id hash找到对应的shard。

下面看一下是如何做到实时可见的?

数据节点接收协调节点请求的入口为:TransportSingleShardAction.ShardTransportHandler# messageReceived:

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@Override
public void messageReceived(final Request request, final TransportChannel channel, Task task) throws Exception {
if (logger.isTraceEnabled()) {
logger.trace("executing [{}] on shard [{}]", request, request.internalShardId);
}
asyncShardOperation(request, request.internalShardId, new ChannelActionListener<>(channel, transportShardAction, request));
}

具体执行是在子类TransportGetAction#asyncShardOperation中:

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@Override
protected void asyncShardOperation(GetRequest request, ShardId shardId, ActionListener<GetResponse> listener) throws IOException {
IndexService indexService = indicesService.indexServiceSafe(shardId.getIndex());
IndexShard indexShard = indexService.getShard(shardId.id());
// 关于realtime可以看一下官方文档
// https://www.elastic.co/guide/en/elasticsearch/reference/current/docs-get.html
if (request.realtime()) { // we are not tied to a refresh cycle here anyway
super.asyncShardOperation(request, shardId, listener);
} else {
indexShard.awaitShardSearchActive(b -> {
try {
super.asyncShardOperation(request, shardId, listener);
} catch (Exception ex) {
listener.onFailure(ex);
}
});
}
}

TransportGetAction#asyncShardOperation获取文档最终调用的是:

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@Override
protected GetResponse shardOperation(GetRequest request, ShardId shardId) {
IndexService indexService = indicesService.indexServiceSafe(shardId.getIndex());
IndexShard indexShard = indexService.getShard(shardId.id());

// 关于realtime、refresh可以看一下官方文档
// https://www.elastic.co/guide/en/elasticsearch/reference/current/docs-get.html
if (request.refresh() && !request.realtime()) {
indexShard.refresh("refresh_flag_get");
}

GetResult result = indexShard.getService().get(request.id(), request.storedFields(),
request.realtime(), request.version(), request.versionType(), request.fetchSourceContext());
return new GetResponse(result);
}

shardOperation先检查是否需要refresh,然后调用indexShard.getService().get()读取数据并存储到GetResult中。读取及过滤 在ShardGetService#get()函数中,调用:
GetResult getResult = innerGet(……);
获取结果。GetResult类用于存储读取的真实数据内容。核心的数据读取实现在ShardGetService#innerGet(……)函数中:

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private GetResult innerGet(String id, String[] gFields, boolean realtime, long version, VersionType versionType,
long ifSeqNo, long ifPrimaryTerm, FetchSourceContext fetchSourceContext) {
fetchSourceContext = normalizeFetchSourceContent(fetchSourceContext, gFields);

// 调用Engine获取数据
Engine.GetResult get = indexShard.get(new Engine.Get(realtime, realtime, id)
.version(version).versionType(versionType).setIfSeqNo(ifSeqNo).setIfPrimaryTerm(ifPrimaryTerm));
assert get.isFromTranslog() == false || realtime : "should only read from translog if realtime enabled";
if (get.exists() == false) {
get.close();
}

if (get == null || get.exists() == false) {
return new GetResult(shardId.getIndexName(), id, UNASSIGNED_SEQ_NO, UNASSIGNED_PRIMARY_TERM, -1, false, null, null, null);
}

try {
// 获取返回结果
// break between having loaded it from translog (so we only have _source), and having a document to load
return innerGetLoadFromStoredFields(id, gFields, fetchSourceContext, get, mapperService);
} finally {
get.close();
}
}


//对指定的field、source进行过滤(source过滤只支持对字段),
//把结果存于GetResult对象中
private GetResult innerGetLoadFromStoredFields(String id, String[] storedFields, FetchSourceContext fetchSourceContext,
Engine.GetResult get, MapperService mapperService) {
assert get.exists() : "method should only be called if document could be retrieved";

// check first if stored fields to be loaded don't contain an object field
DocumentMapper docMapper = mapperService.documentMapper();
if (storedFields != null) {
for (String field : storedFields) {
Mapper fieldMapper = docMapper.mappers().getMapper(field);
if (fieldMapper == null) {
if (docMapper.mappers().objectMappers().get(field) != null) {
// Only fail if we know it is a object field, missing paths / fields shouldn't fail.
throw new IllegalArgumentException("field [" + field + "] isn't a leaf field");
}
}
}
}

Map<String, DocumentField> documentFields = null;
Map<String, DocumentField> metadataFields = null;
BytesReference source = null;
DocIdAndVersion docIdAndVersion = get.docIdAndVersion();
// force fetching source if we read from translog and need to recreate stored fields
boolean forceSourceForComputingTranslogStoredFields = get.isFromTranslog() && storedFields != null &&
Stream.of(storedFields).anyMatch(f -> TranslogLeafReader.ALL_FIELD_NAMES.contains(f) == false);
FieldsVisitor fieldVisitor = buildFieldsVisitors(storedFields,
forceSourceForComputingTranslogStoredFields ? FetchSourceContext.FETCH_SOURCE : fetchSourceContext);
if (fieldVisitor != null) {
try {
docIdAndVersion.reader.document(docIdAndVersion.docId, fieldVisitor);
} catch (IOException e) {
throw new ElasticsearchException("Failed to get id [" + id + "]", e);
}
source = fieldVisitor.source();

// in case we read from translog, some extra steps are needed to make _source consistent and to load stored fields
if (get.isFromTranslog()) {
// Fast path: if only asked for the source or stored fields that have been already provided by TranslogLeafReader,
// just make source consistent by reapplying source filters from mapping (possibly also nulling the source)
if (forceSourceForComputingTranslogStoredFields == false) {
try {
source = indexShard.mapperService().documentMapper().sourceMapper().applyFilters(source, null);
} catch (IOException e) {
throw new ElasticsearchException("Failed to reapply filters for [" + id + "] after reading from translog", e);
}
} else {
// Slow path: recreate stored fields from original source
assert source != null : "original source in translog must exist";
SourceToParse sourceToParse = new SourceToParse(shardId.getIndexName(), id, source, XContentHelper.xContentType(source),
fieldVisitor.routing());
ParsedDocument doc = indexShard.mapperService().documentMapper().parse(sourceToParse);
assert doc.dynamicMappingsUpdate() == null : "mapping updates should not be required on already-indexed doc";
// update special fields
doc.updateSeqID(docIdAndVersion.seqNo, docIdAndVersion.primaryTerm);
doc.version().setLongValue(docIdAndVersion.version);

// retrieve stored fields from parsed doc
fieldVisitor = buildFieldsVisitors(storedFields, fetchSourceContext);
for (IndexableField indexableField : doc.rootDoc().getFields()) {
IndexableFieldType fieldType = indexableField.fieldType();
if (fieldType.stored()) {
FieldInfo fieldInfo = new FieldInfo(indexableField.name(), 0, false, false, false, IndexOptions.NONE,
DocValuesType.NONE, -1, Collections.emptyMap(), 0, 0, 0, false);
StoredFieldVisitor.Status status = fieldVisitor.needsField(fieldInfo);
if (status == StoredFieldVisitor.Status.YES) {
if (indexableField.numericValue() != null) {
fieldVisitor.objectField(fieldInfo, indexableField.numericValue());
} else if (indexableField.binaryValue() != null) {
fieldVisitor.binaryField(fieldInfo, indexableField.binaryValue());
} else if (indexableField.stringValue() != null) {
fieldVisitor.objectField(fieldInfo, indexableField.stringValue());
}
} else if (status == StoredFieldVisitor.Status.STOP) {
break;
}
}
}
// retrieve source (with possible transformations, e.g. source filters
source = fieldVisitor.source();
}
}

// put stored fields into result objects
if (!fieldVisitor.fields().isEmpty()) {
fieldVisitor.postProcess(mapperService::fieldType);
documentFields = new HashMap<>();
metadataFields = new HashMap<>();
for (Map.Entry<String, List<Object>> entry : fieldVisitor.fields().entrySet()) {
if (mapperService.isMetadataField(entry.getKey())) {
metadataFields.put(entry.getKey(), new DocumentField(entry.getKey(), entry.getValue()));
} else {
documentFields.put(entry.getKey(), new DocumentField(entry.getKey(), entry.getValue()));
}
}
}
}

if (source != null) {
// apply request-level source filtering
if (fetchSourceContext.fetchSource() == false) {
source = null;
} else if (fetchSourceContext.includes().length > 0 || fetchSourceContext.excludes().length > 0) {
Map<String, Object> sourceAsMap;
// TODO: The source might be parsed and available in the sourceLookup but that one uses unordered maps so different.
// Do we care?
Tuple<XContentType, Map<String, Object>> typeMapTuple = XContentHelper.convertToMap(source, true);
XContentType sourceContentType = typeMapTuple.v1();
sourceAsMap = typeMapTuple.v2();
sourceAsMap = XContentMapValues.filter(sourceAsMap, fetchSourceContext.includes(), fetchSourceContext.excludes());
try {
source = BytesReference.bytes(XContentFactory.contentBuilder(sourceContentType).map(sourceAsMap));
} catch (IOException e) {
throw new ElasticsearchException("Failed to get id [" + id + "] with includes/excludes set", e);
}
}
}

return new GetResult(shardId.getIndexName(), id, get.docIdAndVersion().seqNo, get.docIdAndVersion().primaryTerm,
get.version(), get.exists(), source, documentFields, metadataFields);
}

下面看一下InternalEngine的读取过程:

InternalEngine#get过程会加读锁。处理realtime选项,如果为true,则先判断是否有数据可以刷盘,然后调用Searcher进行读取。Searcher是对IndexSearcher的封装。

从ES 5.x开始不会从translog中读取,只从Lucene中读。realtime的实现机制变成依靠refresh实现。参考官方链接:https://github.com/elastic/elasticsearch/pull/20102

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@Override
public GetResult get(Get get, DocumentMapper mapper, Function<Engine.Searcher, Engine.Searcher> searcherWrapper) {
assert Objects.equals(get.uid().field(), IdFieldMapper.NAME) : get.uid().field();
try (ReleasableLock ignored = readLock.acquire()) {
ensureOpen();
// 处理realtime选项,判断是否需要刷盘
if (get.realtime()) {
final VersionValue versionValue;
// versionMap中的值是写入索引的时候添加的,不会写到磁盘中
try (Releasable ignore = versionMap.acquireLock(get.uid().bytes())) {
// we need to lock here to access the version map to do this truly in RT
versionValue = getVersionFromMap(get.uid().bytes());
}
if (versionValue != null) {
if (versionValue.isDelete()) {
return GetResult.NOT_EXISTS;
}
if (get.versionType().isVersionConflictForReads(versionValue.version, get.version())) {
throw new VersionConflictEngineException(shardId, get.id(),
get.versionType().explainConflictForReads(versionValue.version, get.version()));
}
if (get.getIfSeqNo() != SequenceNumbers.UNASSIGNED_SEQ_NO && (
get.getIfSeqNo() != versionValue.seqNo || get.getIfPrimaryTerm() != versionValue.term
)) {
throw new VersionConflictEngineException(shardId, get.id(),
get.getIfSeqNo(), get.getIfPrimaryTerm(), versionValue.seqNo, versionValue.term);
}
if (get.isReadFromTranslog()) {
// this is only used for updates - API _GET calls will always read form a reader for consistency
// the update call doesn't need the consistency since it's source only + _parent but parent can go away in 7.0
if (versionValue.getLocation() != null) {
try {
final Translog.Operation operation = translog.readOperation(versionValue.getLocation());
if (operation != null) {
return getFromTranslog(get, (Translog.Index) operation, mapper, searcherWrapper);
}
} catch (IOException e) {
maybeFailEngine("realtime_get", e); // lets check if the translog has failed with a tragic event
throw new EngineException(shardId, "failed to read operation from translog", e);
}
} else {
trackTranslogLocation.set(true);
}
}
assert versionValue.seqNo >= 0 : versionValue;
//执行刷盘操作
refreshIfNeeded("realtime_get", versionValue.seqNo);
}
// 调用Searcher读取数据
return getFromSearcher(get, acquireSearcher("realtime_get", SearcherScope.INTERNAL, searcherWrapper));
} else {
// we expose what has been externally expose in a point in time snapshot via an explicit refresh
return getFromSearcher(get, acquireSearcher("get", SearcherScope.EXTERNAL, searcherWrapper));
}
}
}

小结

  • GET是根据Document _id 哈希找到对应的shard的。
  • 根据Document _id查询的实时可见是通过依靠refresh实现的。

参考资料:
《Elasticsearch源码解析与优化实战》

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