[GitHub] [hudi] prashantwason commented on a change in pull request #4640: [HUDI-3225] [RFC-45] for async metadata indexing

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prashantwason commented on a change in pull request #4640:
URL: https://github.com/apache/hudi/pull/4640#discussion_r812687856



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File path: rfc/rfc-45/rfc-45.md
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+
+# 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

Review comment:
       +1




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