add is-write-index flag to aliases (#30942)

This commit adds the is-write-index flag for aliases.
It allows requests to set the flag, and responses to display the flag.
It does not validate and/or affect any indexing/getting/updating behavior
of Elasticsearch -- this will be done in a follow-up PR.

docs/reference/indices/aliases.asciidoc

@@ -244,6 +244,94 @@ GET /alias2/_search?q=user:kimchy&routing=2,3
 // CONSOLE
 // TEST[continued]
 
+[float]
+[[aliases-write-index]]
+==== Write Index
+
+It is possible to associate the index pointed to by an alias as the write index.
+When specified, all index and update requests against an alias that point to multiple
+indices will attempt to resolve to the one index that is the write index.
+Only one index per alias can be assigned to be the write index at a time. If no write index is specified
+and there are multiple indices referenced by an alias, then writes will not be allowed.
+
+It is possible to specify an index associated with an alias as a write index using both the aliases API
+and index creation API.
+
+[source,js]
+--------------------------------------------------
+POST /_aliases
+{
+    "actions" : [
+        {
+            "add" : {
+                 "index" : "test",
+                 "alias" : "alias1",
+                 "is_write_index" : true
+            }
+        }
+    ]
+}
+--------------------------------------------------
+// CONSOLE
+// TEST[s/^/PUT test\n/]
+
+In this example, we associate the alias `alias1` to both `test` and `test2`, where
+`test` will be the index chosen for writing to.
+
+[source,js]
+--------------------------------------------------
+PUT /alias1/_doc/1
+{
+    "foo": "bar"
+}
+--------------------------------------------------
+// CONSOLE
+// TEST[continued]
+
+The new document that was indexed to `/alias1/_doc/1` will be indexed as if it were
+`/test/_doc/1`.
+
+[source,js]
+--------------------------------------------------
+GET /test/_doc/1
+--------------------------------------------------
+// CONSOLE
+// TEST[continued]
+
+To swap which index is the write index for an alias, the Aliases API can be leveraged to
+do an atomic swap. The swap is not dependent on the ordering of the actions.
+
+[source,js]
+--------------------------------------------------
+POST /_aliases
+{
+    "actions" : [
+        {
+            "add" : {
+                 "index" : "test",
+                 "alias" : "alias1",
+                 "is_write_index" : true
+            }
+        }, {
+            "add" : {
+                 "index" : "test2",
+                 "alias" : "alias1",
+                 "is_write_index" : false
+            }
+        }
+    ]
+}
+--------------------------------------------------
+// CONSOLE
+// TEST[s/^/PUT test\nPUT test2\n/]
+
+[IMPORTANT]
+=====================================
+Aliases that do not explicitly set `is_write_index: true` for an index, and
+only reference one index, will have that referenced index behave as if it is the write index
+until an additional index is referenced. At that point, there will be no write index and
+writes will be rejected.
+=====================================
 
 [float]
 [[alias-adding]]

rest-api-spec/src/main/resources/rest-api-spec/test/indices.create/10_basic.yml

@@ -79,14 +79,37 @@
       indices.get_alias:
         index: test_index
 
-  - match: {test_index.aliases.test_alias: {}}
   - match: {test_index.aliases.test_blias.search_routing: b}
   - match: {test_index.aliases.test_blias.index_routing: b}
   - is_false: test_index.aliases.test_blias.filter
   - match: {test_index.aliases.test_clias.filter.term.field: value}
   - is_false: test_index.aliases.test_clias.index_routing
   - is_false: test_index.aliases.test_clias.search_routing
 
+---
+"Create index with write aliases":
+  - skip:
+      version: " - 6.99.99"
+      reason: is_write_index is not implemented in ES <= 6.x
+  - do:
+      indices.create:
+        index: test_index
+        body:
+          aliases:
+            test_alias: {}
+            test_blias:
+              is_write_index: false
+            test_clias:
+              is_write_index: true
+
+  - do:
+      indices.get_alias:
+        index: test_index
+
+  - is_false: test_index.aliases.test_alias.is_write_index
+  - is_false: test_index.aliases.test_blias.is_write_index
+  - is_true: test_index.aliases.test_clias.is_write_index
+
 ---
 "Create index with no type mappings":
   - do:

server/src/main/java/org/elasticsearch/action/admin/indices/alias/Alias.java

@@ -20,6 +20,7 @@
 package org.elasticsearch.action.admin.indices.alias;
 
 import org.elasticsearch.ElasticsearchGenerationException;
+import org.elasticsearch.Version;
 import org.elasticsearch.common.Nullable;
 import org.elasticsearch.common.ParseField;
 import org.elasticsearch.common.Strings;
@@ -49,6 +50,7 @@ public class Alias implements Streamable, ToXContentFragment {
     private static final ParseField ROUTING = new ParseField("routing");
     private static final ParseField INDEX_ROUTING = new ParseField("index_routing", "indexRouting", "index-routing");
     private static final ParseField SEARCH_ROUTING = new ParseField("search_routing", "searchRouting", "search-routing");
+    private static final ParseField IS_WRITE_INDEX = new ParseField("is_write_index");
 
     private String name;
 
@@ -61,6 +63,9 @@ public class Alias implements Streamable, ToXContentFragment {
     @Nullable
     private String searchRouting;
 
+    @Nullable
+    private Boolean writeIndex;
+
     private Alias() {
 
     }
@@ -167,6 +172,21 @@ public Alias searchRouting(String searchRouting) {
         return this;
     }
 
+    /**
+     * @return the write index flag for the alias
+     */
+    public Boolean writeIndex() {
+        return writeIndex;
+    }
+
+    /**
+     *  Sets whether an alias is pointing to a write-index
+     */
+    public Alias writeIndex(@Nullable Boolean writeIndex) {
+        this.writeIndex = writeIndex;
+        return this;
+    }
+
     /**
      * Allows to read an alias from the provided input stream
      */
@@ -182,6 +202,11 @@ public void readFrom(StreamInput in) throws IOException {
         filter = in.readOptionalString();
         indexRouting = in.readOptionalString();
         searchRouting = in.readOptionalString();
+        if (in.getVersion().onOrAfter(Version.V_7_0_0_alpha1)) {
+            writeIndex = in.readOptionalBoolean();
+        } else {
+            writeIndex = null;
+        }
     }
 
     @Override
@@ -190,6 +215,9 @@ public void writeTo(StreamOutput out) throws IOException {
         out.writeOptionalString(filter);
         out.writeOptionalString(indexRouting);
         out.writeOptionalString(searchRouting);
+        if (out.getVersion().onOrAfter(Version.V_7_0_0_alpha1)) {
+            out.writeOptionalBoolean(writeIndex);
+        }
     }
 
     /**
@@ -219,6 +247,10 @@ public static Alias fromXContent(XContentParser parser) throws IOException {
                 } else if (SEARCH_ROUTING.match(currentFieldName, parser.getDeprecationHandler())) {
                     alias.searchRouting(parser.text());
                 }
+            } else if (token == XContentParser.Token.VALUE_BOOLEAN) {
+                if (IS_WRITE_INDEX.match(currentFieldName, parser.getDeprecationHandler())) {
+                    alias.writeIndex(parser.booleanValue());
+                }
             }
         }
         return alias;
@@ -245,6 +277,8 @@ public XContentBuilder toXContent(XContentBuilder builder, Params params) throws
             }
         }
 
+        builder.field(IS_WRITE_INDEX.getPreferredName(), writeIndex);
+
         builder.endObject();
         return builder;
     }

server/src/main/java/org/elasticsearch/action/admin/indices/alias/IndicesAliasesRequest.java

@@ -20,6 +20,7 @@
 package org.elasticsearch.action.admin.indices.alias;
 
 import org.elasticsearch.ElasticsearchGenerationException;
+import org.elasticsearch.Version;
 import org.elasticsearch.action.ActionRequestValidationException;
 import org.elasticsearch.action.AliasesRequest;
 import org.elasticsearch.action.support.IndicesOptions;
@@ -84,6 +85,7 @@ public static class AliasActions implements AliasesRequest, Writeable, ToXConten
         private static final ParseField ROUTING = new ParseField("routing");
         private static final ParseField INDEX_ROUTING = new ParseField("index_routing", "indexRouting", "index-routing");
         private static final ParseField SEARCH_ROUTING = new ParseField("search_routing", "searchRouting", "search-routing");
+        private static final ParseField IS_WRITE_INDEX = new ParseField("is_write_index");
 
         private static final ParseField ADD = new ParseField("add");
         private static final ParseField REMOVE = new ParseField("remove");
@@ -179,6 +181,7 @@ private static ObjectParser<AliasActions, Void> parser(String name, Supplier<Ali
             ADD_PARSER.declareField(AliasActions::routing, XContentParser::text, ROUTING, ValueType.INT);
             ADD_PARSER.declareField(AliasActions::indexRouting, XContentParser::text, INDEX_ROUTING, ValueType.INT);
             ADD_PARSER.declareField(AliasActions::searchRouting, XContentParser::text, SEARCH_ROUTING, ValueType.INT);
+            ADD_PARSER.declareField(AliasActions::writeIndex, XContentParser::booleanValue, IS_WRITE_INDEX, ValueType.BOOLEAN);
         }
         private static final ObjectParser<AliasActions, Void> REMOVE_PARSER = parser(REMOVE.getPreferredName(), AliasActions::remove);
         private static final ObjectParser<AliasActions, Void> REMOVE_INDEX_PARSER = parser(REMOVE_INDEX.getPreferredName(),
@@ -215,6 +218,7 @@ private static ObjectParser<AliasActions, Void> parser(String name, Supplier<Ali
         private String routing;
         private String indexRouting;
         private String searchRouting;
+        private Boolean writeIndex;
 
         public AliasActions(AliasActions.Type type) {
             this.type = type;
@@ -231,6 +235,9 @@ public AliasActions(StreamInput in) throws IOException {
             routing = in.readOptionalString();
             searchRouting = in.readOptionalString();
             indexRouting = in.readOptionalString();
+            if (in.getVersion().onOrAfter(Version.V_7_0_0_alpha1)) {
+                writeIndex = in.readOptionalBoolean();
+            }
         }
 
         @Override
@@ -242,6 +249,9 @@ public void writeTo(StreamOutput out) throws IOException {
             out.writeOptionalString(routing);
             out.writeOptionalString(searchRouting);
             out.writeOptionalString(indexRouting);
+            if (out.getVersion().onOrAfter(Version.V_7_0_0_alpha1)) {
+                out.writeOptionalBoolean(writeIndex);
+            }
         }
 
         /**
@@ -401,6 +411,18 @@ public AliasActions filter(QueryBuilder filter) {
             }
         }
 
+        public AliasActions writeIndex(Boolean writeIndex) {
+            if (type != AliasActions.Type.ADD) {
+                throw new IllegalArgumentException("[is_write_index] is unsupported for [" + type + "]");
+            }
+            this.writeIndex = writeIndex;
+            return this;
+        }
+
+        public Boolean writeIndex() {
+            return writeIndex;
+        }
+
         @Override
         public String[] aliases() {
             return aliases;

server/src/main/java/org/elasticsearch/action/admin/indices/alias/IndicesAliasesRequestBuilder.java

@@ -130,6 +130,18 @@ public IndicesAliasesRequestBuilder addAlias(String index, String alias, QueryBu
         return this;
     }
 
+    /**
+     * Adds an alias to the index.
+     *
+     * @param index         The index
+     * @param alias         The alias
+     * @param writeIndex    write index flag
+     */
+    public IndicesAliasesRequestBuilder addAlias(String index, String alias, boolean writeIndex) {
+        request.addAliasAction(AliasActions.add().index(index).alias(alias).writeIndex(writeIndex));
+        return this;
+    }
+
     /**
      * Removes an alias from the index.
      *

server/src/main/java/org/elasticsearch/action/admin/indices/alias/TransportIndicesAliasesAction.java

@@ -100,7 +100,8 @@ protected void masterOperation(final IndicesAliasesRequest request, final Cluste
                 switch (action.actionType()) {
                 case ADD:
                     for (String alias : concreteAliases(action, state.metaData(), index)) {
-                        finalActions.add(new AliasAction.Add(index, alias, action.filter(), action.indexRouting(), action.searchRouting()));
+                        finalActions.add(new AliasAction.Add(index, alias, action.filter(), action.indexRouting(),
+                            action.searchRouting(), action.writeIndex()));
                     }
                     break;
                 case REMOVE:

server/src/main/java/org/elasticsearch/action/admin/indices/rollover/TransportRolloverAction.java

@@ -196,7 +196,7 @@ public void onFailure(Exception e) {
     static IndicesAliasesClusterStateUpdateRequest prepareRolloverAliasesUpdateRequest(String oldIndex, String newIndex,
                                                                                        RolloverRequest request) {
         List<AliasAction> actions = unmodifiableList(Arrays.asList(
-                new AliasAction.Add(newIndex, request.getAlias(), null, null, null),
+                new AliasAction.Add(newIndex, request.getAlias(), null, null, null, null),
                 new AliasAction.Remove(oldIndex, request.getAlias())));
         final IndicesAliasesClusterStateUpdateRequest updateRequest = new IndicesAliasesClusterStateUpdateRequest(actions)
             .ackTimeout(request.ackTimeout())