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docs/transformer-selection.md

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+## Transformer selection strategy
+
+The TransformRegistry uses t-config to choose which Transformer will be
+used. A transformer definition contains a supported list of source and
+target Media Types. This is used for the most basic selection. It is further
+refined by checking that the definition also supports transform options (the
+parameters) that have been supplied in a transform request.
+
+~~~
+Transformer 1 defines options: Op1, Op2
+Transformer 2 defines options: Op1, Op2, Op3, Op4
+
+Transform request provides values for options: Op2, Op3
+~~~
+If we assume both transformers support the required source and target Media
+Types, Transformer 2 will be selected because it knows about all the supplied
+options. The definition may also specify that some options are required or
+grouped. If any members of an optional group are supplied, all required
+members of that group become required.
+
+The configuration may impose a source file size limit resulting in the
+selection of a different transformer. Size limits are normally added to avoid
+the transforms consuming too many resources.
+
+The configuration may also specify a priority which will be used in
+Transformer selection if there are a number of possible transformers. The
+highest priority is the one with the lowest number.