diff --git a/benchmarks/parsetostruct.upb_table.c b/benchmarks/parsetostruct.upb_table.c
index e387de65aa..7614b3088b 100644
--- a/benchmarks/parsetostruct.upb_table.c
+++ b/benchmarks/parsetostruct.upb_table.c
@@ -3,12 +3,14 @@
 
 #include "upb_data.h"
 #include "upb_def.h"
+#include "upb_parse.h"
 
 static struct upb_symtab *s;
 static upb_strptr str;
 static struct upb_msgdef *def;
 static upb_msg *msgs[NUM_MESSAGES];
-static struct upb_msgparser *mp;
+static upb_parser *parser;
+static upb_msgsink *sink;
 
 static bool initialize()
 {
@@ -47,7 +49,8 @@ static bool initialize()
     fprintf(stderr, "Error reading " MESSAGE_FILE "\n");
     return false;
   }
-  mp = upb_msgparser_new(def);
+  parser = upb_parser_new(def);
+  sink = upb_msgsink_new(def);
   return true;
 }
 
@@ -57,17 +60,19 @@ static void cleanup()
     upb_msg_unref(msgs[i], def);
   upb_string_unref(str);
   upb_symtab_unref(s);
-  upb_msgparser_free(mp);
+  upb_parser_free(parser);
+  upb_msgsink_free(sink);
 }
 
 static size_t run(int i)
 {
   struct upb_status status = UPB_STATUS_INIT;
   upb_msg *msg = msgs[i%NUM_MESSAGES];
-  upb_msgparser_reset(mp, msg);
+  upb_msgsink_reset(sink, msg);
+  upb_parser_reset(parser, upb_msgsink_sink(sink));
   upb_msg_clear(msg, def);
-  upb_msgparser_parse(mp, str, &status);
-  if(!upb_ok(&status)) {
+  size_t parsed = upb_parser_parse(parser, str, &status);
+  if(!upb_ok(&status) || parsed != upb_strlen(str)) {
     fprintf(stderr, "Parse error: %s\n", status.msg);
     return 0;
   }
diff --git a/src/upb_data.c b/src/upb_data.c
index 3dc39f56d9..4fc1844bdc 100644
--- a/src/upb_data.c
+++ b/src/upb_data.c
@@ -282,18 +282,35 @@ void _upb_msg_free(upb_msg *msg, struct upb_msgdef *md)
   free(msg);
 }
 
+void upb_msg_parsestr(upb_msg *msg, struct upb_msgdef *md, upb_strptr str,
+                      struct upb_status *status)
+{
+  upb_parser *p = upb_parser_new(md);
+  upb_msgsink *s = upb_msgsink_new(md);
+
+  upb_msgsink_reset(s, msg);
+  upb_parser_reset(p, upb_msgsink_sink(s));
+  upb_msg_clear(msg, md);
+  upb_parser_parse(p, str, status);
+
+  upb_parser_free(p);
+  upb_msgsink_free(s);
+}
+
 
-/* Parsing.  ******************************************************************/
+/* upb_msgsrc  ****************************************************************/
 
-struct upb_msgparser_frame {
+/* upb_msgsink  ***************************************************************/
+
+struct upb_msgsink_frame {
   upb_msg *msg;
   struct upb_msgdef *md;
 };
 
-struct upb_msgparser {
-  struct upb_cbparser *s;
-  bool merge;
-  struct upb_msgparser_frame stack[UPB_MAX_NESTING], *top;
+struct upb_msgsink {
+  upb_sink base;
+  struct upb_msgdef *toplevel_msgdef;
+  struct upb_msgsink_frame stack[UPB_MAX_NESTING], *top;
 };
 
 /* Helper function that returns a pointer to where the next value for field "f"
@@ -321,24 +338,26 @@ static union upb_value_ptr get_value_ptr(upb_msg *msg, struct upb_fielddef *f)
   return p;
 }
 
-// Callbacks for the stream parser.
+// Callbacks for upb_sink.
 // TODO: implement these in terms of public interfaces.
 
-static bool value_cb(void *udata, struct upb_fielddef *f, union upb_value val)
+static upb_sink_status _upb_msgsink_valuecb(upb_sink *s, struct upb_fielddef *f,
+                                            union upb_value val)
 {
-  struct upb_msgparser *mp = udata;
-  upb_msg *msg = mp->top->msg;
+  upb_msgsink *ms = (upb_msgsink*)s;
+  upb_msg *msg = ms->top->msg;
   union upb_value_ptr p = get_value_ptr(msg, f);
   upb_msg_sethas(msg, f);
   upb_value_write(p, val, f->type);
-  return true;
+  return UPB_SINK_CONTINUE;
 }
 
-static bool str_cb(void *udata, struct upb_fielddef *f, upb_strptr str,
-                   int32_t start, uint32_t end)
+static upb_sink_status _upb_msgsink_strcb(upb_sink *s, struct upb_fielddef *f,
+                                          upb_strptr str,
+                                          int32_t start, uint32_t end)
 {
-  struct upb_msgparser *mp = udata;
-  upb_msg *msg = mp->top->msg;
+  upb_msgsink *ms = (upb_msgsink*)s;
+  upb_msg *msg = ms->top->msg;
   union upb_value_ptr p = get_value_ptr(msg, f);
   upb_msg_sethas(msg, f);
   if(end > upb_strlen(str)) abort();  /* TODO: support streaming. */
@@ -348,13 +367,13 @@ static bool str_cb(void *udata, struct upb_fielddef *f, upb_strptr str,
     *p.str = upb_string_new();
   }
   upb_strcpylen(*p.str, upb_string_getrobuf(str) + start, end - start);
-  return true;
+  return UPB_SINK_CONTINUE;
 }
 
-static void start_cb(void *udata, struct upb_fielddef *f)
+static upb_sink_status _upb_msgsink_startcb(upb_sink *s, struct upb_fielddef *f)
 {
-  struct upb_msgparser *mp = udata;
-  upb_msg *oldmsg = mp->top->msg;
+  upb_msgsink *ms = (upb_msgsink*)s;
+  upb_msg *oldmsg = ms->top->msg;
   union upb_value_ptr p = get_value_ptr(oldmsg, f);
 
   if(upb_isarray(f) || !upb_msg_has(oldmsg, f)) {
@@ -368,50 +387,50 @@ static void start_cb(void *udata, struct upb_fielddef *f)
     upb_msg_sethas(oldmsg, f);
   }
 
-  mp->top++;
-  mp->top->msg = *p.msg;
+  ms->top++;
+  ms->top->msg = *p.msg;
+  return UPB_SINK_CONTINUE;
 }
 
-static void end_cb(void *udata)
+static upb_sink_status _upb_msgsink_endcb(upb_sink *s)
 {
-  struct upb_msgparser *mp = udata;
-  mp->top--;
+  upb_msgsink *ms = (upb_msgsink*)s;
+  ms->top--;
+  return UPB_SINK_CONTINUE;
 }
 
-/* Externally-visible functions for the msg parser. */
+static upb_sink_callbacks _upb_msgsink_vtbl = {
+  _upb_msgsink_valuecb,
+  _upb_msgsink_strcb,
+  _upb_msgsink_startcb,
+  _upb_msgsink_endcb
+};
 
-struct upb_msgparser *upb_msgparser_new(struct upb_msgdef *def)
-{
-  struct upb_msgparser *mp = malloc(sizeof(struct upb_msgparser));
-  mp->s = upb_cbparser_new(def, value_cb, str_cb, start_cb, end_cb);
-  return mp;
-}
+//
+// External upb_msgsink interface.
+//
 
-void upb_msgparser_reset(struct upb_msgparser *s, upb_msg *msg)
+upb_msgsink *upb_msgsink_new(struct upb_msgdef *md)
 {
-  upb_cbparser_reset(s->s, s);
-  s->top = s->stack;
-  s->top->msg = msg;
+  upb_msgsink *ms = malloc(sizeof(*ms));
+  upb_sink_init(&ms->base, &_upb_msgsink_vtbl);
+  ms->toplevel_msgdef = md;
+  return ms;
 }
 
-void upb_msgparser_free(struct upb_msgparser *s)
+void upb_msgsink_free(upb_msgsink *sink)
 {
-  upb_cbparser_free(s->s);
-  free(s);
+  free(sink);
 }
 
-void upb_msg_parsestr(upb_msg *msg, struct upb_msgdef *md, upb_strptr str,
-                      struct upb_status *status)
+upb_sink *upb_msgsink_sink(upb_msgsink *sink)
 {
-  struct upb_msgparser *mp = upb_msgparser_new(md);
-  upb_msgparser_reset(mp, msg);
-  upb_msg_clear(msg, md);
-  upb_msgparser_parse(mp, str, status);
-  upb_msgparser_free(mp);
+  return &sink->base;
 }
 
-size_t upb_msgparser_parse(struct upb_msgparser *s, upb_strptr str,
-                           struct upb_status *status)
+void upb_msgsink_reset(upb_msgsink *ms, upb_msg *msg)
 {
-  return upb_cbparser_parse(s->s, str, status);
+  ms->top = ms->stack;
+  ms->top->msg = msg;
+  ms->top->md = ms->toplevel_msgdef;
 }
diff --git a/src/upb_data.h b/src/upb_data.h
index 98243ce578..4fc9e8d5b8 100644
--- a/src/upb_data.h
+++ b/src/upb_data.h
@@ -26,6 +26,7 @@
 #include "upb.h"
 #include "upb_atomic.h"
 #include "upb_def.h"
+#include "upb_sink.h"
 
 #ifdef __cplusplus
 extern "C" {
@@ -513,18 +514,47 @@ INLINE void upb_msg_clear(upb_msg *msg, struct upb_msgdef *md) {
   memset(msg->data, 0, md->set_flags_bytes);
 }
 
-/* Parsing ********************************************************************/
-
+// A convenience function for parsing an entire protobuf all at once, without
+// having to worry about setting up the appropriate objects.
 void upb_msg_parsestr(upb_msg *msg, struct upb_msgdef *md, upb_strptr str,
                       struct upb_status *status);
 
-struct upb_msgparser *upb_msgparser_new(struct upb_msgdef *def);
-void upb_msgparser_free(struct upb_msgparser *mp);
 
-void upb_msgparser_reset(struct upb_msgparser *mp, upb_msg *m);
+/* upb_msgsrc *****************************************************************/
+
+// A upb_msgsrc can push the data of a upb_msg to a upb_sink.
+struct upb_msgsrc;
+typedef struct upb_msgsrc upb_msgsrc;
+
+// Allocate and free a msgsrc, respectively.
+upb_msgsrc *upb_msgsrc_new();
+void upb_msgsrc_free(upb_msgsrc *src);
+
+// Resets the msgsrc for the given msg, msgdef, and sink.  This must be
+// called before upb_msgsrc_produce().
+void upb_msgsrc_reset(upb_msgsrc *src, upb_msg *msg, struct upb_msgdef *md,
+                      upb_sink *sink);
+
+// Pushes data from the upb_msgsrc to the sink that was provided at the last
+// reset.  Returns true if the sink is finished, or false if it is suspended.
+bool upb_msgsrc_produce(upb_msgsrc *src);
+
+
+/* upb_msgsink ****************************************************************/
+
+// A upb_msgsink can accept the data from a source and write it into a message.
+struct upb_msgsink;
+typedef struct upb_msgsink upb_msgsink;
+
+// Allocate and free a msgsink, respectively.
+upb_msgsink *upb_msgsink_new(struct upb_msgdef *md);
+void upb_msgsink_free(upb_msgsink *sink);
+
+// Returns the upb_sink (like an upcast).
+upb_sink *upb_msgsink_sink(upb_msgsink *sink);
 
-size_t upb_msgparser_parse(struct upb_msgparser *mp, upb_strptr str,
-                           struct upb_status *status);
+// Resets the msgsink for the given msg.
+void upb_msgsink_reset(upb_msgsink *sink, upb_msg *msg);
 
 #ifdef __cplusplus
 }  /* extern "C" */
diff --git a/src/upb_parse.c b/src/upb_parse.c
index 101792a638..577fa5c37f 100644
--- a/src/upb_parse.c
+++ b/src/upb_parse.c
@@ -310,57 +310,48 @@ static const uint8_t *upb_parse_value(const uint8_t *buf, const uint8_t *end,
 #undef CASE
 }
 
-struct upb_cbparser_frame {
+struct upb_parser_frame {
   struct upb_msgdef *msgdef;
   size_t end_offset;  // For groups, 0.
 };
 
-struct upb_cbparser {
+struct upb_parser {
   // Immutable state of the parser.
   struct upb_msgdef *toplevel_msgdef;
-  upb_value_cb  value_cb;
-  upb_str_cb    str_cb;
-  upb_start_cb  start_cb;
-  upb_end_cb    end_cb;
+  upb_sink *sink;
 
   // State pertaining to a particular parse (resettable).
   // Stack entries store the offset where the submsg ends (for groups, 0).
-  struct upb_cbparser_frame stack[UPB_MAX_NESTING], *top, *limit;
+  struct upb_parser_frame stack[UPB_MAX_NESTING], *top, *limit;
   size_t completed_offset;
   void *udata;
 };
 
-struct upb_cbparser *upb_cbparser_new(struct upb_msgdef *msgdef,
-                                      upb_value_cb valuecb, upb_str_cb strcb,
-                                      upb_start_cb startcb, upb_end_cb endcb)
+upb_parser *upb_parser_new(struct upb_msgdef *msgdef)
 {
-  struct upb_cbparser *p = malloc(sizeof(struct upb_cbparser));
+  upb_parser *p = malloc(sizeof(*p));
   p->toplevel_msgdef = msgdef;
-  p->value_cb = valuecb;
-  p->str_cb = strcb;
-  p->start_cb = startcb;
-  p->end_cb = endcb;
   p->limit = &p->stack[UPB_MAX_NESTING];
   return p;
 }
 
-void upb_cbparser_free(struct upb_cbparser *p)
+void upb_parser_free(upb_parser *p)
 {
   free(p);
 }
 
-void upb_cbparser_reset(struct upb_cbparser *p, void *udata)
+void upb_parser_reset(upb_parser *p, upb_sink *sink)
 {
   p->top = p->stack;
   p->completed_offset = 0;
-  p->udata = udata;
+  p->sink = sink;
   p->top->msgdef = p->toplevel_msgdef;
   // The top-level message is not delimited (we can keep receiving data for it
   // indefinitely), so we treat it like a group.
   p->top->end_offset = 0;
 }
 
-static const void *get_msgend(struct upb_cbparser *p, const uint8_t *start)
+static const void *get_msgend(upb_parser *p, const uint8_t *start)
 {
   if(p->top->end_offset > 0)
     return start + (p->top->end_offset - p->completed_offset);
@@ -385,7 +376,7 @@ INLINE bool upb_check_type(upb_wire_type_t wt, upb_field_type_t ft) {
  * Pushes a new stack frame for a submessage with the given len (which will
  * be zero if the submessage is a group).
  */
-static const uint8_t *push(struct upb_cbparser *p, const uint8_t *start,
+static const uint8_t *push(upb_parser *p, const uint8_t *start,
                            uint32_t submsg_len, struct upb_fielddef *f,
                            struct upb_status *status)
 {
@@ -396,11 +387,11 @@ static const uint8_t *push(struct upb_cbparser *p, const uint8_t *start,
                UPB_MAX_NESTING);
     return NULL;
   }
-  struct upb_cbparser_frame *frame = p->top;
+  struct upb_parser_frame *frame = p->top;
   frame->end_offset = p->completed_offset + submsg_len;
   frame->msgdef = upb_downcast_msgdef(f->def);
 
-  if(p->start_cb) p->start_cb(p->udata, f);
+  upb_sink_onstart(p->sink, f);
   return get_msgend(p, start);
 }
 
@@ -408,16 +399,15 @@ static const uint8_t *push(struct upb_cbparser *p, const uint8_t *start,
  * Pops a stack frame, returning a pointer for where the next submsg should
  * end (or a pointer that is out of range for a group).
  */
-static const void *pop(struct upb_cbparser *p, const uint8_t *start)
+static const void *pop(upb_parser *p, const uint8_t *start)
 {
-  if(p->end_cb) p->end_cb(p->udata);
+  upb_sink_onend(p->sink);
   p->top--;
   return get_msgend(p, start);
 }
 
 
-size_t upb_cbparser_parse(struct upb_cbparser *p, upb_strptr str,
-                          struct upb_status *status)
+size_t upb_parser_parse(upb_parser *p, upb_strptr str, struct upb_status *status)
 {
   // buf is our current offset, moves from start to end.
   const uint8_t *buf = (uint8_t*)upb_string_getrobuf(str);
@@ -429,12 +419,7 @@ size_t upb_cbparser_parse(struct upb_cbparser *p, upb_strptr str,
 
   const uint8_t *submsg_end = get_msgend(p, start);
   struct upb_msgdef *msgdef = p->top->msgdef;
-  bool keep_going = true;
-
-  // Make local copies so optimizer knows they won't change.
-  const upb_str_cb str_cb = p->str_cb;
-  const upb_value_cb value_cb = p->value_cb;
-  void *const udata = p->udata;
+  upb_sink_status sink_status = UPB_SINK_CONTINUE;
 
   // We need to check the status of operations that can fail, but we do so as
   // late as possible to avoid introducing branches that have to wait on
@@ -443,7 +428,7 @@ size_t upb_cbparser_parse(struct upb_cbparser *p, upb_strptr str,
 #define CHECK_STATUS() do { if(!upb_ok(status)) goto err; } while(0)
 
   // Main loop: executed once per tag/field pair.
-  while(keep_going && buf < end) {
+  while(sink_status == UPB_SINK_CONTINUE && buf < end) {
     // Parse/handle tag.
     struct upb_tag tag;
     buf = parse_tag(buf, end, &tag, status);
@@ -476,8 +461,8 @@ size_t upb_cbparser_parse(struct upb_cbparser *p, upb_strptr str,
       } else {
         if(f && upb_isstringtype(f->type)) {
           int32_t str_start = buf - start;
-          keep_going =
-              str_cb(udata, f, str, str_start, str_start + delim_len);
+          sink_status =
+              upb_sink_onstr(p->sink, f, str, str_start, str_start + delim_len);
         } // else { TODO: packed arrays }
         // If field was not found, it is skipped silently.
         buf = delim_end;  // Could be >end.
@@ -493,7 +478,7 @@ size_t upb_cbparser_parse(struct upb_cbparser *p, upb_strptr str,
         buf = upb_parse_value(buf, end, f->type, upb_value_addrof(&val),
                               status);
         CHECK_STATUS();  // Checking upb_parse_value().
-        keep_going = value_cb(udata, f, val);
+        sink_status = upb_sink_onvalue(p->sink, f, val);
       }
     }
     CHECK_STATUS();
diff --git a/src/upb_parse.h b/src/upb_parse.h
index 9cc997420c..9afa96ca27 100644
--- a/src/upb_parse.h
+++ b/src/upb_parse.h
@@ -24,106 +24,31 @@
 extern "C" {
 #endif
 
-/* Callback parser callbacks. *************************************************/
+/* upb_parser *****************************************************************/
 
-// The value callback is called when a regular value (ie. not a string or
-// submessage) is encountered which was defined in the upb_msgdef.  The client
-// returns true to continue the parse or false to halt it.
-//
-// Note that this callback can be called several times in a row for a single
-// call to tag_cb in the case of packed arrays.
-typedef bool (*upb_value_cb)(void *udata, struct upb_fielddef *f,
-                             union upb_value val);
-
-// The string callback is called when a string that was defined in the
-// upb_msgdef is parsed.  "str" is the protobuf data that is being parsed (NOT
-// the string in question); "start" and "end" are the start and end offset of
-// the string we parsed *within* str.  The data is supplied this way to give
-// you the opportunity to reference this data instead of copying it (perhaps
-// using upb_strslice), or to minimize copying if it is unavoidable.
-//
-// Note that if you are parsing in a streaming fashion, start could be <0 and
-// "end" could be >upb_strlen(str).
-typedef bool (*upb_str_cb)(void *udata, struct upb_fielddef *f, upb_strptr str,
-                           int32_t start, uint32_t end);
-
-// The start and end callbacks are called when a submessage begins and ends,
-// respectively.
-typedef void (*upb_start_cb)(void *udata, struct upb_fielddef *f);
-typedef void (*upb_end_cb)(void *udata);
+// A upb_parser parses the binary protocol buffer format, writing the data it
+// parses to a upb_sink.
+struct upb_parser;
+typedef struct upb_parser upb_parser;
 
-/* Callback parser interface. *************************************************/
-
-// Allocates and frees a upb_cbparser, respectively.  Callbacks may be NULL,
-// in which case they will be skipped.
-struct upb_cbparser *upb_cbparser_new(struct upb_msgdef *md,
-                                      upb_value_cb valuecb, upb_str_cb strcb,
-                                      upb_start_cb startcb, upb_end_cb endcb);
-void upb_cbparser_free(struct upb_cbparser *p);
+// Allocates and frees a upb_parser, respectively.
+upb_parser *upb_parser_new(struct upb_msgdef *md);
+void upb_parser_free(upb_parser *p);
 
 // Resets the internal state of an already-allocated parser.  This puts it in a
 // state where it has not seen any data, and expects the next data to be from
 // the beginning of a new protobuf.  Parsers must be reset before they can be
-// used.  A parser can be reset multiple times.  udata will be passed as the
-// first argument to callbacks.
-void upb_cbparser_reset(struct upb_cbparser *p, void *udata);
+// used.  A parser can be reset multiple times.
+void upb_parser_reset(upb_parser *p, upb_sink *sink);
 
-// Parses up to len bytes of protobuf data out of buf, calling the appropriate
-// callbacks as values are parsed.
-//
-// The function returns a status indicating the success of the operation.  Data
-// is parsed until no more data can be read from buf, or a user callback
-// returns false, or an error occurs.
-//
-// The function returns the number of bytes consumed.  Note that this can be
-// greater than len in the case that a string was recognized that spans beyond
-// the end of the currently provided data.
+// Parses protobuf data out of str, returning how much data was parsed.  The
+// next call to upb_parser_parse should begin with the first byte that was
+// not parsed.  "status" indicates whether an error occurred.
 //
-// The next call to upb_parse must be the first byte after buf + retval, even in
-// the case that retval > len.
-//
-// TODO: see if we can provide the following guarantee efficiently:
+// TODO: provide the following guarantee:
 //   retval will always be >= len. */
-size_t upb_cbparser_parse(struct upb_cbparser *p, upb_strptr str,
-                          struct upb_status *status);
-
-/* Pick parser interface. ************************************************/
-
-// The pick parser provides a convenient interface for extracting a given set
-// of fields from a protobuf.  This is especially useful in the case that you
-// want only a few fields from a large protobuf, because the pick parser can be
-// much more efficient by aggressively skipping data and stopping when it has
-// all the fields you asked for.  The requested fields may be nested
-// submessages of the top-level message.
-//
-// The selection parser currently does not yet support repeated fields -- this
-// would involve either letting the user specify an index of the record they
-// wanted, or repeatedly delivering values for the same field number.  The
-// latter would make it impossible to bail out of processing a message early,
-// because there could always be more values for that field.
-//
-// This parser is layered on top of the callback parser.
-
-// Callbacks for the pick parser.  The semantics are the same as for the
-// callback parser, excet that field numbers are provided instead of msgdefs
-// and fieldefs.
-typedef void (*upb_pp_value_cb)(void *udata, int fieldnum, union upb_value val);
-typedef void (*upb_pp_str_cb)(void *udata, int fieldnum, uint8_t *str,
-                              size_t avail_len, size_t total_len);
-
-// The pickparser methods all have the same semantics as the cbparser, except
-// that there are no start or end callbacks and the constructor needs a list
-// of fields.  The fields are in dotted notation, so "foo.bar" expects that the
-// top-level message contains a field foo, which contains a field bar.  The
-// new function will return NULL if any of the field names are invalid, or are
-// repeated fields.
-struct upb_pickparser *upb_pickparser_new(struct upb_msgdef *msgdef,
-                                          char *fields[]);
-void upb_pickparser_free(struct upb_pickparser *p);
-void upb_pickparser_reset(struct upb_pickparser *p,
-                          bool found[], union upb_value vals[]);
-size_t upb_pickparser_parse(struct upb_pickparser *p, upb_strptr str,
-                            struct upb_status *status);
+size_t upb_parser_parse(upb_parser *p, upb_strptr str,
+                        struct upb_status *status);
 
 #ifdef __cplusplus
 }  /* extern "C" */
diff --git a/src/upb_sink.h b/src/upb_sink.h
new file mode 100644
index 0000000000..17e1e1d640
--- /dev/null
+++ b/src/upb_sink.h
@@ -0,0 +1,117 @@
+/*
+ * upb - a minimalist implementation of protocol buffers.
+ *
+ * Copyright (c) 2010 Joshua Haberman.  See LICENSE for details.
+ *
+ * upb_sink is a general purpose interface for pushing the contents of a
+ * protobuf from one component to another in a streaming fashion.  We call the
+ * component that calls a upb_sink a "source".  By "pushing" we mean that the
+ * source calls into the sink; the opposite (where a sink calls into the
+ * source) is known as "pull".  In the push model the source gets the main
+ * loop; in a pull model the sink does.
+ *
+ * This interface is used as general-purpose glue in upb.  For example, the
+ * parser interface works by implementing a source.  Likewise the serialization
+ * simply implements a sink.  Copying one protobuf to another is just a matter
+ * of using one message as a source and another as a sink.
+ *
+ * In terms of efficiency, we would generally expect "push" to be faster if the
+ * source had more state to track, and "pull" to be faster if the sink had more
+ * state.  The reason is that whoever has the main loop can keep state on the
+ * stack (and possibly even in callee-save registers), whereas the the
+ * component that is "called into" always needs to reload its state from
+ * memory.
+ *
+ * In terms of programming complexity, it is easier and simpler to have the
+ * main loop, because you can store state in local variables.
+ *
+ * So the assumption inherent in using the push model is that sources are
+ * generally more complicated and stateful than consumers.  For example, in the
+ * parser case, it has to deal with malformed input and associated errors; in
+ * comparison, the serializer deals with known-good input.
+ */
+
+#ifndef UPB_SINK_H
+#define UPB_SINK_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// Each of the upb_sink callbacks returns a status of this type.
+typedef enum {
+  // The normal case, where the consumer wants to continue consuming.
+  UPB_SINK_CONTINUE,
+
+  // The consumer has consumed the current value, but it wants to stop for now.
+  // When the producer is next invoked, it should resume at the next value.
+  UPB_SINK_SUSPEND,
+
+  // The consumer wants to skip to the end of the current submessage and
+  // continue consuming.  If we are at the top-level, the rest of the
+  // data is discarded.
+  UPB_SINK_SKIP
+} upb_sink_status;
+
+
+typedef struct {
+  struct upb_sink_callbacks *vtbl;
+} upb_sink;
+
+/* upb_sink callbacks *********************************************************/
+
+// The value callback is called when a regular value (ie. not a string or
+// submessage) is pushed.
+typedef upb_sink_status (*upb_value_cb)(upb_sink *s, struct upb_fielddef *f,
+                                        union upb_value val);
+
+// The string callback is called when a string is pushed.  "str" is the string
+// in which the data lives, but it may contain more data than the effective
+// string.  "start" and "end" indicate the substring of "str" that is the
+// effective string.  If "start" is <0, this string is a continuation of the
+// previous string for this field.  If end > upb_strlen(str) then there is more
+// data to follow for this string.  "end" can also be used as a hint for how
+// much data follows, but this is only a hint and is not guaranteed.
+//
+// The data is supplied this way to give you the opportunity to reference this
+// data instead of copying it (perhaps using upb_strslice), or to minimize
+// copying if it is unavoidable.
+typedef upb_sink_status (*upb_str_cb)(upb_sink *s, struct upb_fielddef *f,
+                                      upb_strptr str,
+                                      int32_t start, uint32_t end);
+
+// The start and end callbacks are called when a submessage begins and ends,
+// respectively.
+typedef upb_sink_status (*upb_start_cb)(upb_sink *s, struct upb_fielddef *f);
+typedef upb_sink_status (*upb_end_cb)(upb_sink *s);
+
+
+/* upb_sink implementation *************************************************/
+
+typedef struct upb_sink_callbacks {
+  upb_value_cb value_cb;
+  upb_str_cb str_cb;
+  upb_start_cb start_cb;
+  upb_end_cb end_cb;
+} upb_sink_callbacks;
+
+// We could potentially define these later to also be capable of calling a C++
+// virtual method instead of doing the virtual dispatch manually.  This would
+// make it possible to write C++ sinks in a more natural style.  We could have
+// a flag in upb_sink defining whether it is a C sink or a C++ one.
+#define upb_sink_onvalue(s, f, val) s->vtbl->value_cb(s, f, val)
+#define upb_sink_onstr(s, f, str, start, end) s->vtbl->str_cb(s, f, str, start, end)
+#define upb_sink_onstart(s, f) s->vtbl->start_cb(s, f)
+#define upb_sink_onend(s) s->vtbl->end_cb(s)
+
+// Initializes a plain C visitor with the given vtbl.  The visitor must have
+// been allocated separately.
+INLINE void upb_sink_init(upb_sink *s, upb_sink_callbacks *vtbl) {
+  s->vtbl = vtbl;
+}
+
+#ifdef __cplusplus
+}  /* extern "C" */
+#endif
+
+#endif