Reindexing data in Elasticsearch - Step-by-Step Execution

Concept Flow - Reindexing data

Start Reindex Request

↓

Read Documents from Source Index

↓

Apply Optional Transformations

↓

Write Documents to Destination Index

↓

Check for Errors

↓

Complete or Retry

Reindexing reads documents from one index, optionally changes them, then writes to another index.

Execution Sample

Elasticsearch

{
  "source": { "index": "old_index" },
  "dest": { "index": "new_index" }
}

This JSON tells Elasticsearch to copy all documents from 'old_index' to 'new_index'.

Execution Table

Step	Action	Source Index Docs Read	Docs Written to Destination	Status
1	Start reindex	0	0	Request sent
2	Read batch of 100 docs	100	0	Reading documents
3	Write batch of 100 docs	100	100	Documents written
4	Read next batch of 100 docs	200	100	Reading documents
5	Write next batch of 100 docs	200	200	Documents written
6	Read final batch of 50 docs	250	200	Reading documents
7	Write final batch of 50 docs	250	250	Documents written
8	Reindex complete	250	250	Success

💡 All 250 documents read from source and written to destination index successfully.

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	Final
docs_read	0	100	200	250	250
docs_written	0	0	100	200	250
status	idle	reading	reading	reading	success

Key Moments - 3 Insights

Why do docs_written lag behind docs_read during the process?

Because Elasticsearch reads a batch of documents first (docs_read increases), then writes them to the destination (docs_written increases). See steps 2 and 3 in execution_table.

What happens if an error occurs during writing?

The reindex process pauses or retries depending on error handling. This is not shown in the current successful trace but would appear as a different status in the execution_table.

Can we transform documents during reindexing?

Yes, you can add scripts or queries to modify documents between reading and writing. This example shows a simple copy without changes.

Visual Quiz - 3 Questions

Look at the execution_table, how many documents have been written after step 5?

A 100

B 150

C 200

D 250

💡 Hint

Check the 'Docs Written to Destination' column at step 5.

At which step does the reindexing process complete successfully?

A Step 8

B Step 7

C Step 6

D Step 5

💡 Hint

Look for the row with status 'Success' in the execution_table.

If the source index had 500 documents instead of 250, how would the docs_read value change after step 4?

A 400

B 200

C 250

D 500

💡 Hint

Step 4 shows docs_read after reading two batches of 100 documents each.

Concept Snapshot

Reindexing copies documents from one Elasticsearch index to another.
Use a JSON body with 'source' and 'dest' indexes.
Elasticsearch reads documents in batches, optionally transforms them, then writes to the new index.
Monitor progress by tracking docs read and written.
Errors can pause or retry the process.
Useful for changing mappings or reorganizing data.

Full Transcript

Reindexing data in Elasticsearch means copying documents from one index to another. The process starts by sending a reindex request. Elasticsearch reads documents from the source index in batches. After reading each batch, it writes those documents to the destination index. This continues until all documents are copied. The execution table shows each step: starting, reading batches, writing batches, and completing successfully. Variables like docs_read and docs_written track progress. Beginners often wonder why docs_written lags behind docs_read; this is because reading happens before writing each batch. Transformations can be added between reading and writing but are not shown here. The process ends when all documents are copied without errors.