diff --git a/src/journal/src/Journal/Types.hs b/src/journal/src/Journal/Types.hs
index b7d724ac..74419cd5 100644
--- a/src/journal/src/Journal/Types.hs
+++ b/src/journal/src/Journal/Types.hs
@@ -48,8 +48,9 @@ pARTITION_COUNT :: Int
 pARTITION_COUNT = 3
 
 data Journal' = Journal'
-  { jTermBuffers :: {-# UNPACK #-} !(Vector ByteBuffer)
-  , jMetadata    :: {-# UNPACK #-} !ByteBuffer
+  { jTermBuffers   :: {-# UNPACK #-} !(Vector ByteBuffer)
+  , jMetadata      :: {-# UNPACK #-} !ByteBuffer
+  , jPositionLimit :: {-# UNPACK #-} !(IORef Int) -- ???
   }
 
 data JMetadata = JMetadata
@@ -92,8 +93,8 @@ lOG_META_DATA_LENGTH = lOG_PAGE_SIZE_OFFSET
 ------------------------------------------------------------------------
 
 rawTail :: ByteBuffer -> Int -> IO Int64
-rawTail metadataBuffer partitionIndex =
-  readInt64OffArrayIx metadataBuffer
+rawTail meta partitionIndex =
+  readInt64OffArrayIx meta
     (tERM_TAIL_COUNTERS_OFFSET + (sizeOf (8 :: Int64) * partitionIndex))
 
 termId :: Int64 -> Int32
@@ -154,11 +155,57 @@ termLength meta = readInt32OffArrayIx meta lOG_TERM_LENGTH_OFFSET
 pageSize :: ByteBuffer -> IO Int32
 pageSize meta = readInt32OffArrayIx meta lOG_PAGE_SIZE_OFFSET
 
+-- | The number of bits to shift when multiplying or dividing by the term buffer
+-- length.
+positionBitsToShift :: Int32 -> Int
+positionBitsToShift termBufferLength =
+  case termBufferLength of
+    65536      {- 64   * 1024 -}        -> 16
+    131072     {- 128  * 1024 -}        -> 17
+    262144     {- 256  * 1024 -}        -> 18
+    524288     {- 512  * 1024 -}        -> 19
+    1048576    {- 1024 * 1024 -}        -> 20
+    2097152    {- 2    * 1024 * 1024 -} -> 21
+    4194304    {- 4    * 1024 * 1024 -} -> 22
+    8388608    {- 8    * 1024 * 1024 -} -> 23
+    16777216   {- 16   * 1024 * 1024 -} -> 24
+    33554432   {- 32   * 1024 * 1024 -} -> 25
+    67108864   {- 64   * 1024 * 1024 -} -> 26
+    134217728  {- 128  * 1024 * 1024 -} -> 27
+    268435456  {- 256  * 1024 * 1024 -} -> 28
+    536870912  {- 512  * 1024 * 1024 -} -> 29
+    1073741824 {- 1024 * 1024 * 1024 -} -> 30
+    _otherwise ->
+      error ("positionBitsToShift: invalid term buffer length: " ++
+             show (termBufferLength))
+
 ------------------------------------------------------------------------
 
+-- | Rotate to the next partition in sequence for the current term id.
+nextPartitionIndex :: Int32 -> Int32
+nextPartitionIndex currentIndex =
+  (currentIndex + 1) `mod` fromIntegral pARTITION_COUNT
+
+-- | Calculate the partition index to be used given the initial term and active
+-- term ids.
+indexByTerm :: Int32 -> Int32 -> Int32
+indexByTerm initTermId activeTermId =
+  (activeTermId - initTermId) `mod` fromIntegral pARTITION_COUNT
+
+-- | Caluclate the partition index based on number of terms that have passed.
 indexByTermCount :: Int32 -> Int
 indexByTermCount termCount = fromIntegral termCount `mod` pARTITION_COUNT
 
+-- | Calculate the partition index given a stream position.
+indexByPosition :: Int64 -> Int -> Int
+indexByPosition pos posBitsToShift = fromIntegral $
+  (pos `shiftR` posBitsToShift) `mod` fromIntegral pARTITION_COUNT
+
+-- | Compute the current position in absolute number of bytes.
+computePosition = undefined
+
+-- | Compute the current position in absolute number of bytes for the beginning
+-- of a term.
 computeTermBeginPosition :: Int32 -> Int32 -> Int32 -> Int64
 computeTermBeginPosition activeTermId posBitsToShift initTermId =
   let
@@ -168,11 +215,31 @@ computeTermBeginPosition activeTermId posBitsToShift initTermId =
   in
     termCount `shiftL` fromIntegral posBitsToShift
 
+-- | Compute the term id from a position.
+computeTermIdFromPosition = undefined
+
+-- | Compute the total length of a log file given the term length.
+computeLogLength = undefined
+
+-- | Rotate the log and update the tail counter for the new term. This function
+-- is thread safe.
 rotateLog :: ByteBuffer -> Int32 -> Int32 -> IO Bool
 rotateLog meta termCount termId0 = do
-  -- XXX:
-  undefined
-  casActiveTermCount meta termCount (termCount + 1)
+  go
+  casActiveTermCount meta termCount nextTermCount
+  where
+    nextTermId     = termId0   + 1
+    nextTermCount  = termCount + 1
+    nextIndex      = indexByTermCount nextTermCount
+    expectedTermId = nextTermId - fromIntegral pARTITION_COUNT
+
+    go = do
+      rt <- rawTail meta nextIndex
+      if expectedTermId /= termId rt
+      then return ()
+      else do
+        b <- casRawTail meta nextIndex rt (packTail nextTermId 0)
+        if b then return () else go
 
 ------------------------------------------------------------------------