Every MongoDB 1,815 client takes a URI, and mongosh 403 , Compass 1,815 , the driver, Mongoose 243,355 and the database tools parse it with the same rules.
The scheme decides how hosts are found. mongodb:// takes a literal comma-separated list of hosts and ports. mongodb+srv:// takes one hostname, asks DNS for its SRV records to build the seed list and a TXT record for default options, and turns TLS on implicitly. SRV is what Atlas 1,815 hands you: a cluster can replace nodes without any client changing configuration, at the cost of a DNS round trip and a hard failure where SRV is blocked.
The path segment is the default database — what db means in mongosh — not an access restriction. Credentials authenticate against authSource, which defaults to admin for SCRAM users, so a URI ending /bookshelf with a user created in admin still needs ?authSource=admin. Percent-encode reserved characters: p@ss:word becomes p%40ss%3Aword. Setting appName is the cheapest win below: every slow query in the log and every db.currentOp() entry then names the process behind it.
| Option | Default | Why change it |
|---|---|---|
| maxPoolSize | 100 | lower on serverless or M0 |
| minPoolSize | 0 | pre-warm to avoid first-request latency |
| serverSelectionTimeoutMS | 30000 | fail fast in a health check |
| w | 1 | majority for durability (Read Preference) |
| appName | none | tags queries in logs and currentOp |
URI options and MongoClient constructor options do the same job. Put deployment facts in the URI so operators can change them without a rebuild, and application facts in code.