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BSON's design flaw

In situations where JSON’s convenient, human-readable syntax results in noticeably slower parse times and/or increased memory usage, one might look for the more compact BSON. Aside from sitting smaller in memory, BSON libraries can more easily provide non-owning solutions, which allow them to reference a read-only chunk of BSON data directly, rather than own a copy internally. This has the big selling point of allowing memory-mapped files to be used as a BSON backend, while all operations on the immutable BSON require no copying or ownership. Unfortunately, there exists an under-documented design flaw in BSON which renders it incompatible with common, standard JSON.

Use case

The issue arises as soon as there are top-level arrays. For example, this is valid JSON:

[1, "kitty", "foo"]

Unfortunately, after some more research, this doesn’t map to valid BSON. The BSON mailing list covered this in three threads:

  • On 3/19/10, it was said: “very good point regarding the root document. i think there should be a rev of bson at some point, and we should all participate in what changes, and we should be very careful what changes before making any change”
  • On 11/14/10, this was brought up again: “perhaps this is a design bug”
  • Finally, on 4/13/11, it was resurfaced: “yes. probably was an oversight long ago.”

Each of the threads references a to-be-announced BSON 2.0; since it’s been nearly seven years, this probably isn’t happening. Either way, it’s crucial to note that your data may not be representable in BSON without changes.


If the performance gains still compel you to make the switch to BSON, you might find yourself having to make top-level arrays into nested elements of a singly-keyed map, perhaps specified with a unique identifier.

{"$array": [1, "kitty", "foo"]}

This isn’t a show stopper for me, and it may not be for you, but this should certainly not be swept under the rug. In fact, the official description of BSON seems a bit unfair, in this regard:

... a binary-encoded serialization of JSON-like documents. Like JSON, BSON
supports the embedding of documents and arrays within other documents
and arrays.

It’s not wrong. You can embed documents in arrays. Unfortunately, everything must be in a document.

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