What is the Aggregation Framework in MongoDB?

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The Aggregation Framework is MongoDB's data processing pipeline. Documents flow through a sequence of stages , each transforming the input before passing it on. Common stages — $match (filter), $project (reshape), $group (aggregate), $sort , $limit , $lookup (join). $group — the equivalent of SQL GROUP BY ; supports accumulators like $sum , $avg , $max , $push . $lookup — left-outer join to another collection. Order matters — put $match and $project early to reduce data flowing through later stages; use indexes when possible. Modern preference over Map-Reduce — faster, easier to read, more features. Know the major stages: $match (filter first for performance), $group (aggregation), $lookup (JOIN), $unwind (flatten arrays), $project (reshape), $sort, $limit. Pipeline order matters — $match early reduces work for later stages. $lookup + $unwind is the JOIN pattern. Aggregation replaced Map-Reduce.