CASE 59 · DRIFT · 2023
MongoDB Atlas to DocumentDB, with the apps unchanged.
A mobile app backend with 14 services talking to MongoDB Atlas had a $9,400/mo cluster bill, plus egress charges as the application moved more workload to AWS. We migrated to DocumentDB compatible mode, preserving the MongoDB driver code in the apps.
Mobile app backend
MIGRATION
2023
RESULTS
What changed, by the numbers.
DATABASE COST
−56%
EGRESS COST
−$2.1K/mo
APP CHANGES
CONN. STRING
p99 LATENCY
+8%
HOW IT WENT
The compatibility analysis was the careful work. DocumentDB supports a subset of MongoDB; we ran the application’s query patterns against a DocumentDB compatibility checker and surfaced four queries that needed adjustment (two aggregations using unsupported operators, two index hints that no longer applied).
DMS did the bulk load and continuous replication for the cutover. Application teams updated four queries during a sprint, well in advance of cutover. The connection string switch was the cutover moment — the apps themselves were unchanged.
Bill dropped 56%. Egress charges (which had been growing as more app components moved to AWS) disappeared. p99 latency went up 8% on a few collection scans — well within the SLO for this application. The team kept Atlas for two months as a cold standby before terminating.
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