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Franck Pachot
Franck Pachot

Posted on • Edited on

WiredTigerHS.wt: MongoDB MVCC Durable History Store

#mongodb #wiredtiger #database #mvcc

MongoDB uses the WiredTiger storage engine, which implements Multi‑Version Concurrency Control (MVCC) to provide lock‑free read consistency, similar to many RDBMS. Unlike many RDBMS, it follows a No‑Force/No‑Steal policy: uncommitted changes stay only in memory. They are never written to disk, and committed changes are written later — at checkpoint or when cache eviction needs space — into the WiredTiger table files we have explored in the previous post, persisting only the latest committed version.
MongoDB also maintains recent committed MVCC versions for a specified period in a separate, durable history store (WiredTigerHS.wt). This enables the system to reconstruct snapshots from earlier points in time. In the previous article in this series, I described all WiredTiger files except WiredTigerHS.wt, because it was empty:

ls -l /data/db/WiredTigerHS.wt

-rw-------. 1 root root 4096 Sep 27 11:01 /data/db/WiredTigerHS.wt
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This 4KB file holds no records:

wt -h /data/db dump -j file:WiredTigerHS.wt

{
    "WiredTiger Dump Version" : "1 (12.0.0)",
    "file:WiredTigerHS.wt" : [
        {
            "config" : "access_pattern_hint=none,allocation_size=4KB,app_metadata=,assert=(commit_timestamp=none,durable_timestamp=none,read_timestamp=none,write_timestamp=off),block_allocation=best,block_compressor=snappy,block_manager=default,cache_resident=false,checksum=on,colgroups=,collator=,columns=,dictionary=0,disaggregated=(page_log=),encryption=(keyid=,name=),exclusive=false,extractor=,format=btree,huffman_key=,huffman_value=,ignore_in_memory_cache_size=false,immutable=false,import=(compare_timestamp=oldest_timestamp,enabled=false,file_metadata=,metadata_file=,panic_corrupt=true,repair=false),in_memory=false,ingest=,internal_item_max=0,internal_key_max=0,internal_key_truncate=true,internal_page_max=16KB,key_format=IuQQ,key_gap=10,leaf_item_max=0,leaf_key_max=0,leaf_page_max=32KB,leaf_value_max=64MB,log=(enabled=true),lsm=(auto_throttle=,bloom=,bloom_bit_count=,bloom_config=,bloom_hash_count=,bloom_oldest=,chunk_count_limit=,chunk_max=,chunk_size=,merge_max=,merge_min=),memory_page_image_max=0,memory_page_max=5MB,os_cache_dirty_max=0,os_cache_max=0,prefix_compression=false,prefix_compression_min=4,source=,split_deepen_min_child=0,split_deepen_per_child=0,split_pct=90,stable=,tiered_storage=(auth_token=,bucket=,bucket_prefix=,cache_directory=,local_retention=300,name=,object_target_size=0),type=file,value_format=QQQu,verbose=[],write_timestamp_usage=none",
            "colgroups" : [],
            "indices" : []
        },
        {
            "data" : [            ]
        }
    ]
}
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The file contains only a header block with the configuration metadata. It defines the key and value format:

key_format=IuQQ    
value_format=QQQu
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Those are WiredTiger types: I and Q are integers, respectively 4-byte and 8-byte, and u is a variable-length type, as an array of bytes.

The history store key (IuQQ) includes the table identifier (collection), the key in this table (recordID), the MVCC start timestamp (indicating when this version was current), and a counter (if multiple updates at the same timestamp). Its value (QQQu) contains the MVCC stop timestamp (when the version became obsolete), the durable timestamp (reflecting when the record reached a persistence point, such as a checkpoint), an update type, and the byte array is the BSON representation of the document version. Start and stop timestamps track version visibility for this document version. The durable timestamp shows when a version is safe to remove, supporting features such as rollback-to-stable, replication catch-up, and crash recovery.

To get some records in it, I start MongoDB as a one-member replicaset:

mongod --dbpath /data/db --replSet rs0 --wiredTigerCacheSizeGB 0.25 &  

mongosh --eval '
  rs.initiate( { _id: "rs0", members: [
  {_id: 0, priority: 1, host: "localhost:27017"},
  ]});
'
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I insert five documents and update them, to have two versions of the documents, the current one with { val: "newvalue" } and the previous one with { val: "oldvalue" }:

db.test.drop();  
for (let i = 0; i < 5; i++) {  
    db.test.insertOne({  
        _id: i,  
        val: "oldvalue",  
        filler: "X".repeat(1024)  
    });  
}   
for (let i = 0; i < 5; i++) {  
    db.test.updateOne(  
        { _id: i },  
        { $set: { val: "newvalue" } } // change to whatever new value you want  
    );  
}
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Until a checkpoint or cache eviction occurs, all changes remain in memory (the WiredTiger cache), protected by write-ahead logging (WAL). To get something in the files, I watch the mongod log and wait for a checkpoint:

{"t":{"$date":"2025-09-27T20:33:18.140+00:00"},"s":"I",  "c":"WTCHKPT",  "id":22430,   "ctx":"Checkpointer","msg":"WiredTiger message","attr":{"message":{"ts_sec":1759005198,"ts_usec":140184,"thread":"12233:0x7f908e1f76c0","session_name":"WT_SESSION.checkpoint","category":"WT_VERB_CHECKPOINT_PROGRESS","category_id":7,"verbose_level":"DEBUG_1","verbose_level_id":1,"msg":"saving checkpoint snapshot min: 196, snapshot max: 196 snapshot count: 0, oldest timestamp: (1759005138, 1) , meta checkpoint timestamp: (1759005188, 1) base write gen: 1"}}}
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The durable history storage file size has increased:

ls -alrt WiredTigerHS.wt
-rw-------. 1 root root 20480 Sep 27 20:33 WiredTigerHS.wt
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I stopped mongod to be able to read the files with wt (that I compiled in a Docker container, as in the earlier post) of this series:

pkill mongod
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There are 18 records in the durable history file, and the ones from my collection are visible as I filled a field with a thousand 'X' characters (0x58), so they are easy to spot in a hex/BSON dump:

wt -h /data/db dump file:WiredTigerHS.wt 

WiredTiger Dump (WiredTiger Version 12.0.0)
Format=print
Header
file:WiredTigerHS.wt
access_pattern_hint=none,allocation_size=4KB,app_metadata=,assert=(commit_timestamp=none,durable_timestamp=none,read_timestamp=none,write_timestamp=off),block_allocation=best,block_compressor=snappy,block_manager=default,cache_resident=false,checksum=on,colgroups=,collator=,columns=,dictionary=0,disaggregated=(page_log=),encryption=(keyid=,name=),exclusive=false,extractor=,format=btree,huffman_key=,huffman_value=,ignore_in_memory_cache_size=false,immutable=false,import=(compare_timestamp=oldest_timestamp,enabled=false,file_metadata=,metadata_file=,panic_corrupt=true,repair=false),in_memory=false,ingest=,internal_item_max=0,internal_key_max=0,internal_key_truncate=true,internal_page_max=16KB,key_format=IuQQ,key_gap=10,leaf_item_max=0,leaf_key_max=0,leaf_page_max=32KB,leaf_value_max=64MB,log=(enabled=true),lsm=(auto_throttle=,bloom=,bloom_bit_count=,bloom_config=,bloom_hash_count=,bloom_oldest=,chunk_count_limit=,chunk_max=,chunk_size=,merge_max=,merge_min=),memory_page_image_max=0,memory_page_max=5MB,os_cache_dirty_max=0,os_cache_max=0,prefix_compression=false,prefix_compression_min=4,source=,split_deepen_min_child=0,split_deepen_per_child=0,split_pct=90,stable=,tiered_storage=(auth_token=,bucket=,bucket_prefix=,cache_directory=,local_retention=300,name=,object_target_size=0),type=file,value_format=QQQu,verbose=[],write_timestamp_usage=none
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\aa\81\85\e8h\d8I\f8\ff\ff\df\c6\80
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Enter fullscreen mode Exit fullscreen mode

The first entries are related to some internal collections. Then there are five records for my collection, easy to recognize with the repeated 'X' filler. My five insertOne() didn’t generate history store entries, as new inserts become the current version with no prior state to retain.
History store entries for my collection appear only after updateOne() changed the val fields. Each entry contains MVCC metadata and the full previous BSON document, representing a full before-image of the collection's document, even if only a single field changed. With this dump showing ASCII characters, we can recognize the field names _id, val, and filler, as well as the text values oldvalue and X.

I can decode the metadata outside of the BSON document:

wt -h /data/db dump -j file:WiredTigerHS.wt

{
    "WiredTiger Dump Version" : "1 (12.0.0)",
    "file:WiredTigerHS.wt" : [
        {
            "config" : "access_pattern_hint=none,allocation_size=4KB,app_metadata=,assert=(commit_timestamp=none,durable_timestamp=none,read_timestamp=none,write_timestamp=off),block_allocation=best,block_compressor=snappy,block_manager=default,cache_resident=false,checksum=on,colgroups=,collator=,columns=,dictionary=0,disaggregated=(page_log=),encryption=(keyid=,name=),exclusive=false,extractor=,format=btree,huffman_key=,huffman_value=,ignore_in_memory_cache_size=false,immutable=false,import=(compare_timestamp=oldest_timestamp,enabled=false,file_metadata=,metadata_file=,panic_corrupt=true,repair=false),in_memory=false,ingest=,internal_item_max=0,internal_key_max=0,internal_key_truncate=true,internal_page_max=16KB,key_format=IuQQ,key_gap=10,leaf_item_max=0,leaf_key_max=0,leaf_page_max=32KB,leaf_value_max=64MB,log=(enabled=true),lsm=(auto_throttle=,bloom=,bloom_bit_count=,bloom_config=,bloom_hash_count=,bloom_oldest=,chunk_count_limit=,chunk_max=,chunk_size=,merge_max=,merge_min=),memory_page_image_max=0,memory_page_max=5MB,os_cache_dirty_max=0,os_cache_max=0,prefix_compression=false,prefix_compression_min=4,source=,split_deepen_min_child=0,split_deepen_per_child=0,split_pct=90,stable=,tiered_storage=(auth_token=,bucket=,bucket_prefix=,cache_directory=,local_retention=300,name=,object_target_size=0),type=file,value_format=QQQu,verbose=[],write_timestamp_usage=none",
            "colgroups" : [],
            "indices" : []
        },
        {
            "data" : [
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"value3" : 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u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0058\u0000\u0000"
}
            ]
        }
    ]
}
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We can recognize the format described above. In the last record, "key0" : 42 identifies the collection, "key1": "\u0085" the recordId, and the version was an update (3 is WT_UPDATE_STANDARD), valid from 7554869708709691398 to 7554869708709691403 (logical timestamps). To read the BSON, I can use bsondump to get the BSON content from the value:

wt -h /data/db dump -x file:WiredTigerHS.wt | 
 grep "585858" | # grep by XXX
 cut -c 39-    | # skip to "value3"
 xxd -r -p     | # 
 bsondump --pretty


{
        "_id": {
                "$numberInt": "0"
        },
        "val": "oldvalue",
        "filler": "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"
}
{
        "_id": {
                "$numberInt": "1"
        },
        "val": "oldvalue",
        "filler": "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"
}
{
        "_id": {
                "$numberInt": "2"
        },
        "val": "oldvalue",
        "filler": "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"
}
{
        "_id": {
                "$numberInt": "3"
        },
        "val": "oldvalue",
        "filler": "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"
}
{
        "_id": {
                "$numberInt": "4"
        },
        "val": "oldvalue",
        "filler": "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"
}
2025-09-27T21:09:38.578+0000    5 objects found
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This output shows the exact before-image of the documents.

Conclusion

Durable History, introduced with MongoDB 4.4, replaced the lookaside table, which was a cache overflow, with a persistent history store. It stores the previous versions of documents as a before-image at checkpoint or cache eviction. The durable history is stored in a dedicated B-tree that doesn't impact operations on current data. The key groups the history by the collection's documents and versioning timestamp. The before-image is the full BSON. By default, checkpoints occur every minute and durable history expires after five minutes (minSnapshotHistoryWindowInSeconds).

This design preserves the recent historical record versions across checkpoints and server restarts. It allows a restart to flashback to a previous consistent state, for example, in a disaster recovery where the quorum was lost, as well as serving long‑running snapshot reads without blocking cache evictions.

MVCC implementations vary significantly across database systems. In MongoDB’s WiredTiger storage engine, start and stop timestamps define the visibility window for a document version in a manner conceptually similar to PostgreSQL’s xmin and xmax transaction identifiers, which track when a row becomes valid and when it is superseded. The difference is that WiredTiger uses 64‑bit logical timestamps, which removes the wraparound risk that PostgreSQL must address by periodically freezing transaction IDs.

Historical versions in WiredTiger are generated out of place and not for every update. They are only produced for committed versions, and only at checkpoint or cache eviction events. Because older data is never left in line with live records, MongoDB avoids the accumulation of dead tuples and table bloat found in PostgreSQL, and therefore does not require a process like VACUUM to reclaim space.

The durable history store in WiredTiger resembles the undo tablespace used in Oracle Database, but there are significant differences. Oracle maintains undo as rollback segments for each transaction, storing fine‑grained undo change vectors because its MVCC design is optimized for transactional rollback. MongoDB keeps uncommitted changes entirely in memory, which makes rollback straightforward, and its history store is instead tuned to support read snapshots over time. Another distinction is that Oracle writes undo data for almost every change. In contrast, MongoDB’s history store only records snapshots of committed data at checkpoints or cache evictions, employing a copy-on-write strategy.

Finally, it’s not possible to declare one MVCC implementation as universally better — each involves trade‑offs. Traditional No‑Force/Steal designs, as found in Oracle and PostgreSQL, allow uncommitted changes to be flushed to disk, which relieves memory pressure because any dirty buffer can be evicted. However, this increases disk pressure, as those uncommitted versions must still be stored, replicated to standbys, read again later for cleanup, and then rewritten and replicated once more.
In contrast, MongoDB’s No-Force/No-Steal approach is optimized for short-lived transactions on modern hardware that can horizontally scale: uncommitted changes are kept entirely in RAM. This design ensures that only clean, committed data is written to data files, eliminating the need for post-commit vacuuming or cleanup processes, and keeping on-disk tables free of bloat. However, the trade-off is that long-running transactions may abort if they cannot fit into memory.

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