nsivabalan commented on a change in pull request #4640: URL: https://github.com/apache/hudi/pull/4640#discussion_r824747770
########## File path: rfc/rfc-45/rfc-45.md ########## @@ -0,0 +1,281 @@ +<!-- + Licensed to the Apache Software Foundation (ASF) under one or more + contributor license agreements. See the NOTICE file distributed with + this work for additional information regarding copyright ownership. + The ASF licenses this file to You under the Apache License, Version 2.0 + (the "License"); you may not use this file except in compliance with + the License. You may obtain a copy of the License at + + http://www.apache.org/licenses/LICENSE-2.0 + + Unless required by applicable law or agreed to in writing, software + distributed under the License is distributed on an "AS IS" BASIS, + WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + See the License for the specific language governing permissions and + limitations under the License. +--> + +# RFC-45: Asynchronous Metadata Indexing + +## Proposers + +- @codope +- @manojpec + +## Approvers + +- @nsivabalan +- @vinothchandar + +## Status + +JIRA: [HUDI-2488](https://issues.apache.org/jira/browse/HUDI-2488) + +## Abstract + +Metadata indexing (aka metadata bootstrapping) is the process of creation of one +or more metadata-based indexes, e.g. data partitions to files index, that is +stored in Hudi metadata table. Currently, the metadata table (referred as MDT +hereafter) supports single partition which is created synchronously with the +corresponding data table, i.e. commits are first applied to metadata table +followed by data table. Our goal for MDT is to support multiple partitions to +boost the performance of existing index and records lookup. However, the +synchronous manner of metadata indexing is not very scalable as we add more +partitions to the MDT because the regular writers (writing to the data table) +have to wait until the MDT commit completes. In this RFC, we propose a design to +support asynchronous metadata indexing. + +## Background + +We can read more about the MDT design +in [RFC-15](https://cwiki.apache.org/confluence/display/HUDI/RFC+-+15%3A+HUDI+File+Listing+Improvements) +. Here is a quick summary of the current state (Hudi v0.10.1). MDT is an +internal Merge-on-Read (MOR) table that has a single partition called `files` +which stores the data partitions to files index that is used in file listing. +MDT is co-located with the data table (inside `.hoodie/metadata` directory under +the basepath). In order to handle multi-writer scenario, users configure lock +provider and only one writer can access MDT in read-write mode. Hence, any write +to MDT is guarded by the data table lock. This ensures only one write is +committed to MDT at any point in time and thus guarantees serializability. +However, locking overhead adversely affects the write throughput and will reach +its scalability limits as we add more partitions to the MDT. + +## Goals + +- Support indexing one or more partitions in MDT while regular writers and table + services (such as cleaning or compaction) are in progress. +- Locking to be as lightweight as possible. +- Keep required config changes to a minimum to simplify deployment / upgrade in + production. +- Do not require specific ordering of how writers and table service pipelines + need to be upgraded / restarted. +- If an external long-running process is being used to initialize the index, the + process should be made idempotent so it can handle errors from previous runs. +- To re-initialize the index, make it as simple as running the external + initialization process again without having to change configs. + +## Implementation + +### A new Hudi action: INDEX + +We introduce a new action `index` which will denote the index building process, +the mechanics of which is as follows: + +1. From an external process, users can issue a CREATE INDEX or similar statement + to trigger indexing for an existing table. + 1. This will schedule INDEX action and add + a `<instant_time>.index.requested` to the timeline, which contains the + indexing plan. Index scheduling will also initialize the filegroup for + the partitions for which indexing is planned. The creation of filegroups + will be done within a lock. + 2. From here on, the index building process will continue to build an index + up to instant time `t`, where `t` is the latest completed instant time on + the timeline without any + "holes" i.e. no pending async operations prior to it. Review comment: not necessarily async. it could be regular writes too. in case of multi-writers, there could be a failed commit waiting to be rolled back. ########## File path: rfc/rfc-45/rfc-45.md ########## @@ -0,0 +1,281 @@ +<!-- + Licensed to the Apache Software Foundation (ASF) under one or more + contributor license agreements. See the NOTICE file distributed with + this work for additional information regarding copyright ownership. + The ASF licenses this file to You under the Apache License, Version 2.0 + (the "License"); you may not use this file except in compliance with + the License. You may obtain a copy of the License at + + http://www.apache.org/licenses/LICENSE-2.0 + + Unless required by applicable law or agreed to in writing, software + distributed under the License is distributed on an "AS IS" BASIS, + WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + See the License for the specific language governing permissions and + limitations under the License. +--> + +# RFC-45: Asynchronous Metadata Indexing + +## Proposers + +- @codope +- @manojpec + +## Approvers + +- @nsivabalan +- @vinothchandar + +## Status + +JIRA: [HUDI-2488](https://issues.apache.org/jira/browse/HUDI-2488) + +## Abstract + +Metadata indexing (aka metadata bootstrapping) is the process of creation of one +or more metadata-based indexes, e.g. data partitions to files index, that is +stored in Hudi metadata table. Currently, the metadata table (referred as MDT +hereafter) supports single partition which is created synchronously with the +corresponding data table, i.e. commits are first applied to metadata table +followed by data table. Our goal for MDT is to support multiple partitions to +boost the performance of existing index and records lookup. However, the +synchronous manner of metadata indexing is not very scalable as we add more +partitions to the MDT because the regular writers (writing to the data table) +have to wait until the MDT commit completes. In this RFC, we propose a design to +support asynchronous metadata indexing. + +## Background + +We can read more about the MDT design +in [RFC-15](https://cwiki.apache.org/confluence/display/HUDI/RFC+-+15%3A+HUDI+File+Listing+Improvements) +. Here is a quick summary of the current state (Hudi v0.10.1). MDT is an +internal Merge-on-Read (MOR) table that has a single partition called `files` +which stores the data partitions to files index that is used in file listing. +MDT is co-located with the data table (inside `.hoodie/metadata` directory under +the basepath). In order to handle multi-writer scenario, users configure lock +provider and only one writer can access MDT in read-write mode. Hence, any write +to MDT is guarded by the data table lock. This ensures only one write is +committed to MDT at any point in time and thus guarantees serializability. +However, locking overhead adversely affects the write throughput and will reach +its scalability limits as we add more partitions to the MDT. + +## Goals + +- Support indexing one or more partitions in MDT while regular writers and table + services (such as cleaning or compaction) are in progress. +- Locking to be as lightweight as possible. +- Keep required config changes to a minimum to simplify deployment / upgrade in + production. +- Do not require specific ordering of how writers and table service pipelines + need to be upgraded / restarted. +- If an external long-running process is being used to initialize the index, the + process should be made idempotent so it can handle errors from previous runs. +- To re-initialize the index, make it as simple as running the external + initialization process again without having to change configs. + +## Implementation + +### A new Hudi action: INDEX + +We introduce a new action `index` which will denote the index building process, +the mechanics of which is as follows: + +1. From an external process, users can issue a CREATE INDEX or similar statement + to trigger indexing for an existing table. + 1. This will schedule INDEX action and add + a `<instant_time>.index.requested` to the timeline, which contains the + indexing plan. Index scheduling will also initialize the filegroup for + the partitions for which indexing is planned. The creation of filegroups + will be done within a lock. + 2. From here on, the index building process will continue to build an index + up to instant time `t`, where `t` is the latest completed instant time on + the timeline without any + "holes" i.e. no pending async operations prior to it. + 3. The indexing process will write these out as base files within the + corresponding metadata partition. A metadata partition cannot be used if + there is any pending indexing action against it. As and when indexing is + completed for a partition, then table config (`hoodie.properties`) will + be updated to indicate that partition is available for reads or + synchronous updates. Hudi table config will be the source of truth for + the current state of metadata index. + +2. Any inflight writers (i.e. with instant time `t'` > `t`) will check for any + new indexing request on the timeline prior to preparing to commit. + 1. Such writers will proceed to additionally add log entries corresponding + to each such indexing request into the metadata partition. + 2. There is always a TOCTOU issue here, where the inflight writer may not + see an indexing request that was just added and proceed to commit without + that. We will correct this during indexing action completion. In the + average case, this may not happen and the design has liveness. + +3. When the indexing process is about to complete (i.e. indexing upto + instant `t` is done but before completing indexing commit), it will check for + all completed commit instants after `t` to ensure each of them added entries + per its indexing plan, otherwise simply abort after a configurable timeout. + Let's call this the **indexing check**. So, the indexer will not only write + base files but also ensure that log entries due to instants after `t` are in + the same filegroup i.e. no new filegroup is initialized by writers while + indexing is in progress. + 1. The corner case here would be that the indexing check does not factor in + the inflight writer just about to commit. But given indexing would take + some finite amount of time to go from requested to completion (or we can + add some, configurable artificial delays here say 60 seconds), an + inflight writer, that is just about to commit concurrently, has a very + high chance of seeing the indexing plan and aborting itself. + +We can just introduce a lock for adding events to the timeline and these races +would vanish completely, still providing great scalability and asynchrony for +these processes. The indexer will error out if there is no lock provider +configured. + +### Multi-writer scenario + + + +Let us walkthrough a concrete mutli-writer scenario to understand the above +indexing mechanism. In this scenario, let instant `t0` be the last completed +instant on the timeline. Suppose user triggered index building from an external +process at `t3`. This will create `t3.index.requested` file with the indexing +plan. The plan contains the metadata partitions that need to be created and the +last completed instant, e.g. + +``` +[ + {MetadataPartitionType.FILES.partitionPath(), t0}, + {MetadataPartitionType.BLOOM_FILTER.partitionPath(), t0}, + {MetadataPartitionType.COLUMN_STATS.partitionPath(), t0} +] +``` + +Further, suppose there were two inflight writers Writer1 and Writer2 (with +inflight instants `t1` and `t2` respectively) while the indexing was requested +or inflight. In this case, the writers will check for pending index action and +find a pending instant `t3`. Now, if the metadata index creation is pending, Review comment: In the attached image, I see locks. Would be good to cover whats the critical section for which we acquire lock here for entire design in general. for eg: regular writers when checking for pending indexing? regular writers to check for completed partitions in MDT? (from table config) async indexer while updating the hoodie table config ? etc. something like this. I am not claiming we need to acquire lock for all of above. But a list like this would be good to call it out explicitly. ########## File path: rfc/rfc-45/rfc-45.md ########## @@ -0,0 +1,281 @@ +<!-- + Licensed to the Apache Software Foundation (ASF) under one or more + contributor license agreements. See the NOTICE file distributed with + this work for additional information regarding copyright ownership. + The ASF licenses this file to You under the Apache License, Version 2.0 + (the "License"); you may not use this file except in compliance with + the License. You may obtain a copy of the License at + + http://www.apache.org/licenses/LICENSE-2.0 + + Unless required by applicable law or agreed to in writing, software + distributed under the License is distributed on an "AS IS" BASIS, + WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + See the License for the specific language governing permissions and + limitations under the License. +--> + +# RFC-45: Asynchronous Metadata Indexing + +## Proposers + +- @codope +- @manojpec + +## Approvers + +- @nsivabalan +- @vinothchandar + +## Status + +JIRA: [HUDI-2488](https://issues.apache.org/jira/browse/HUDI-2488) + +## Abstract + +Metadata indexing (aka metadata bootstrapping) is the process of creation of one +or more metadata-based indexes, e.g. data partitions to files index, that is +stored in Hudi metadata table. Currently, the metadata table (referred as MDT +hereafter) supports single partition which is created synchronously with the +corresponding data table, i.e. commits are first applied to metadata table +followed by data table. Our goal for MDT is to support multiple partitions to +boost the performance of existing index and records lookup. However, the +synchronous manner of metadata indexing is not very scalable as we add more +partitions to the MDT because the regular writers (writing to the data table) +have to wait until the MDT commit completes. In this RFC, we propose a design to +support asynchronous metadata indexing. + +## Background + +We can read more about the MDT design +in [RFC-15](https://cwiki.apache.org/confluence/display/HUDI/RFC+-+15%3A+HUDI+File+Listing+Improvements) +. Here is a quick summary of the current state (Hudi v0.10.1). MDT is an +internal Merge-on-Read (MOR) table that has a single partition called `files` +which stores the data partitions to files index that is used in file listing. +MDT is co-located with the data table (inside `.hoodie/metadata` directory under +the basepath). In order to handle multi-writer scenario, users configure lock +provider and only one writer can access MDT in read-write mode. Hence, any write +to MDT is guarded by the data table lock. This ensures only one write is +committed to MDT at any point in time and thus guarantees serializability. +However, locking overhead adversely affects the write throughput and will reach +its scalability limits as we add more partitions to the MDT. + +## Goals + +- Support indexing one or more partitions in MDT while regular writers and table + services (such as cleaning or compaction) are in progress. +- Locking to be as lightweight as possible. +- Keep required config changes to a minimum to simplify deployment / upgrade in + production. +- Do not require specific ordering of how writers and table service pipelines + need to be upgraded / restarted. +- If an external long-running process is being used to initialize the index, the + process should be made idempotent so it can handle errors from previous runs. +- To re-initialize the index, make it as simple as running the external + initialization process again without having to change configs. + +## Implementation + +### A new Hudi action: INDEX + +We introduce a new action `index` which will denote the index building process, +the mechanics of which is as follows: + +1. From an external process, users can issue a CREATE INDEX or similar statement + to trigger indexing for an existing table. + 1. This will schedule INDEX action and add + a `<instant_time>.index.requested` to the timeline, which contains the + indexing plan. Index scheduling will also initialize the filegroup for + the partitions for which indexing is planned. The creation of filegroups + will be done within a lock. + 2. From here on, the index building process will continue to build an index + up to instant time `t`, where `t` is the latest completed instant time on + the timeline without any + "holes" i.e. no pending async operations prior to it. + 3. The indexing process will write these out as base files within the + corresponding metadata partition. A metadata partition cannot be used if + there is any pending indexing action against it. As and when indexing is + completed for a partition, then table config (`hoodie.properties`) will + be updated to indicate that partition is available for reads or + synchronous updates. Hudi table config will be the source of truth for + the current state of metadata index. + +2. Any inflight writers (i.e. with instant time `t'` > `t`) will check for any + new indexing request on the timeline prior to preparing to commit. + 1. Such writers will proceed to additionally add log entries corresponding + to each such indexing request into the metadata partition. + 2. There is always a TOCTOU issue here, where the inflight writer may not + see an indexing request that was just added and proceed to commit without + that. We will correct this during indexing action completion. In the + average case, this may not happen and the design has liveness. + +3. When the indexing process is about to complete (i.e. indexing upto + instant `t` is done but before completing indexing commit), it will check for + all completed commit instants after `t` to ensure each of them added entries + per its indexing plan, otherwise simply abort after a configurable timeout. + Let's call this the **indexing check**. So, the indexer will not only write + base files but also ensure that log entries due to instants after `t` are in + the same filegroup i.e. no new filegroup is initialized by writers while + indexing is in progress. + 1. The corner case here would be that the indexing check does not factor in + the inflight writer just about to commit. But given indexing would take + some finite amount of time to go from requested to completion (or we can + add some, configurable artificial delays here say 60 seconds), an + inflight writer, that is just about to commit concurrently, has a very + high chance of seeing the indexing plan and aborting itself. + +We can just introduce a lock for adding events to the timeline and these races +would vanish completely, still providing great scalability and asynchrony for +these processes. The indexer will error out if there is no lock provider +configured. + +### Multi-writer scenario + + + +Let us walkthrough a concrete mutli-writer scenario to understand the above +indexing mechanism. In this scenario, let instant `t0` be the last completed +instant on the timeline. Suppose user triggered index building from an external +process at `t3`. This will create `t3.index.requested` file with the indexing +plan. The plan contains the metadata partitions that need to be created and the +last completed instant, e.g. + +``` +[ + {MetadataPartitionType.FILES.partitionPath(), t0}, + {MetadataPartitionType.BLOOM_FILTER.partitionPath(), t0}, + {MetadataPartitionType.COLUMN_STATS.partitionPath(), t0} +] +``` + +Further, suppose there were two inflight writers Writer1 and Writer2 (with +inflight instants `t1` and `t2` respectively) while the indexing was requested +or inflight. In this case, the writers will check for pending index action and +find a pending instant `t3`. Now, if the metadata index creation is pending, +which means indexer has already intialized a filegroup, then each writer will +create log files in the same filegroup for the metadata index update. This will +happen within the existing data table lock. + +The indexer runs in a loop until the metadata for data upto `t0` plus the data Review comment: would like to understand the loop here. I thought we will just go for one round and then timeout. will sync up f2f. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: commits-unsubscr...@hudi.apache.org For queries about this service, please contact Infrastructure at: us...@infra.apache.org
